this is an unedited draft reflecting my personal opinions ... ·...

$: This is an unedited draft reflecting my personal opinions ... · C:\work\PROJECTS\HSIS\IHSDM-Multilane\Literature Reviews\1. Lane width\lane width.wpd 1.1 This is an unedited draft$
The terms lane width, roadway width, carriageway or pavement width will be used1

interchangeably.

Earlier drafts of this papers were prepared in the course of a project for UMA2

Engineering (for the new Canadian Geometric Design Guide) and for DELCAN (in ORSAM 98).

“On rural two-lane roads capacities of 10 and 11-fr lanes, expressed as a percentage of3

12-ft lane capacity are 77 and 88 percent, respectively.r four lane undivided highways thesepercentages are 89 and 95 . . .” (Green book, 1984, p. 360). Capacity is the reciprocal value ofthe average minimum headway. Thus, with wider lanes drivers choose shorter headways.

1.1C:\work\PROJECTS\HSIS\IHSDM-Multilane\Literature Reviews\1. Lane width\lane width.wpd

This is an unedited draft reflecting my personal opinions. Ezra Hauer

Lane Width and Safety.1

E. Hauer. Draft , March 7, 2000.2

1. Introduction.

The link between lane width and safety is woven of two principal strands. First, the wider the

lanes the larger will be the average separation between vehicles moving in adjacent lanes. This may

provide a wider buffer to adsorb the small random deviations of vehicles from their intended path.

However, drivers adapt to the road they see. Wider lanes tend to induce somewhat faster travel and

perhaps closer following (as evident in the relationship between lane width and capacity ). Whether3

this complex adaptation to wider lanes benefits safety or whether it harms it, cannot be anticipated

by speculation. Useful information can be extracted only from empirical evidence. As will become

evident, the preconceived notion that wider lanes must be safer has, at times, intruded on the

judgement of researchers. The second strand in the link between safety and lane width is that a wider

lane may provides more room for correction in near-accident circumstances. Thus, e.g., for a narrow

lane a moment’s inattention may lead a vehicle off the edge-drop and onto a gravel shoulder but if

the lane is wider and the shoulder paved the same inattention will still leave the vehicle on the paved

surface. In these near-accident circumstances, it will be difficult to separate between the effect of lane

width, shoulder width, shoulder paving, edge-drops etc.

It is likely that lane width plays a somewhat different role in single and multilane roads. The

lane width requirements for single-lane roads were originally derived from the observation of driver

behaviour. That lane width at which drivers did not feel the need to shift to the right when meeting

and oncoming truck was deemed appropriate. The same criterion may apply to the inner lane of an

undivided multilane lane road, but it does not apply to the other lanes nor to divided roads.

1.5

1.75

2

2.25

2.5

Acc

iden

ts/M

VM

16 18 20 22 24 Carriageway Width [ft]


Figure 1. Non-junction accidents for two-lane roads in Buckhinghampshire.

2. One matter of method.

Before embarking on the survey of what researchers have found, at least one methodological

issue requires airing. Early research relied on the examination of variables one-by-one: Accident rate

was plotted on the vertical axis against lane width (or degree of curve, grade, etc.) on the horizontal

axis. This intuitively appealing practice may lead astray.

To illustrate, consider an early study (Research on Road Traffic, p.411), most likely based on

Coburn, T.M., The relation between accidents and layout on rural roads. International Road Safety

and Traffic Review, Autumn 1962, pp. 15-20) which shows the relationship in Figure 1.

The squares show the accident rates for the four carriageway width categories in which data

has been reported. Thus, carriageways that are 16-17 ft wide had 2.3 accide nts/MVM (MVM=Million

Vehicle Miles) as shown by point A while carriageways 22 to 24 ft wide had an accident rate of 1.7

accidents/MVM as shown by point B. It is tempting to interpret this to mean that widening the

carriageway will reduce the number of accidents.

However, narrower roads usually carry less traffic. Indeed, in this case, the relationship

between carriageway width and average ADT (ADT=Average Daily Traffic) is shown in Figure 2.

0

1000

2000

3000

Ave

rage

Dai

ly T

raffi

c

16 18 20 22 24 Carriageway Width [ft]

0

1

2

Acc

iden

ts/(

mile

-yea

r)

0 1000 2000 3000 Average Daily Traffic (ADT)


Figure 2

Figure 3

It is normal and proper

engineering practice for roads that

carry little traffic to be built and

maintained to lesser standards.

Therefore one may expect not only

carriageway width but also shoulder

width, horizontal curvature, distance

to obstacles, gradients, sight distances,

side access etc., all these to be

associated with ADT. It follows that

the association seen in Figure 1 may

represent not only (or mainly) the

effect of carriageway width but also (perhaps predominantly) the effect of all the other causal factors

that affect accident occurrence and are associated with traffic flow.

How, in this case, accident frequency

and ADT are associated is shown in

Figure 3. The non-linear

relationship between traffic and

accident frequency in Figure 3 is found

in many data sets by many researchers.

It reflect not only the tendency of

design and maintenance standards to

be a function of traffic, but also the

complexities of car following, speed

choice, driver vigilance, accident

severity, inclination to report accidents

etc., all of which vary with the

intensity of traffic.

The squares in Figure 3 are the data from Research on Road Ttraffic to which the smooth

curve 0.003×ADT has been fitted. If this expression correctly represents reality, then the accident0.8

rate is given by accidents/MVM=0.003×ADT / (ADT×365×10 ). This follows from the definition0.8 -6

1

2

3

Acc

iden

ts/M

VM

0 1000 2000 3000 Average Daily Traffic ADT

A

B


Figure 4

of the accident rate. How this accident ra te varies with ADT is shown by the smooth curve in Figure

4.

If then Figure 3 correctly

represents the association between

accident frequency and traffic flow, it

is a logical consequence that the

accident rate must decline with traffic

flow. This decline is a reflection of the

many things that change when traffic

flow changes - design standards, road

maintenance, traffic characteristics,

driver behaviour - not only of the

change in carriageway width. It

follows that what is pictured in Figure

1 as the influence of carriageway width

is in reality only a reflection of the fact that all low traffic flows the accident rate tends to be high for

various reasons, and that narrow carriageways tend to go with low traffic flows. When commenting

on the same report, (Research on Road Ttraffic) say, Roy Jorgensen and Associates say that:

“Therefore, ADT may be affecting the accident rates because it was not held constant for the

analysis”.

The kind of relationship between accident frequency and ADT that is depicted in Figure 3

has been found repeatedly when data was examined (see, e.g., Roy Jorgensen Associates, p.18 or

Zegeer, Deen, et al. 1981 ). Other researchers (see, e.g.,Silyanov , Slatterly & Cleveland 1969 p.

312} report an upward curving relationship, implying that accident rate increases with ADT. Be it

as it may, when the accident rate decreases or increases with ADT and the feature of interest (here

lane width) is associated with ADT, one may not come to conclusions about the effect of that feature

on safety, without separating what part of the change in the accident rate is due to the ADT and what

part is due to the feature of interest.

One could perhaps try to argue that it is still unclear whether the relationship in Figure 3

(accident frequency versus ADT) is the primary one, explaining the curve in Figure 1 as an illusory

artifact, or whether the relationship in Figure 1 is the primary one, and is the main cause of the

curvilinear relationship in Figure 3. To settle this question I use the results recently obtained by

Stewart & Council for two-lane rural roads with 6 ft shoulders in North Carolina and Washington.

0

1

2

3

4

5

Acc

iden

ts/(

mile

-yea

r)

0 5000 10000 15000 20000 Average Daily Traffic

Wa, 24 ft

Wa, 22

NC, 24 ft

NC, 22 ft

0

0.5

1

1.5

2 A

ccid

ents

/(m

ile-y

ear)

0 5000 10000 15000 20000Average Daily Traffic

Wa, 24 ft

Wa, 22

NC, 24 ft

NC, 22 ft


Figure 5. Total accidents Figure 6. Injury Accidents

The model equation is of the form: Accidents/(mile-year)="(ADT) . The estimated parameter values�

are given in Table 1 and the functions depicted in Figures 5 and 6.

Table 1

22 ft. travel way with 6 N=497 � =0.0311 � =0.4733

ft .shoulders North Carolina � =0.083 � =0.5408Totla

Injury

Total

Injury

24 ft. travelway with 6 N=433 � =0.0133 � =0.5775

ft. Shoulders � =0.0072 � =0.5550Total

Injury

Total

Injury

22 ft. travel way with 6 N=222 � =0.000612 � =0.919

ft .shoulders Washington � =0.000112 � =1.033Total

Injury

Total

Injury

24 ft. travelway with 6 N=213 � =0.00760 � =0.607

ft. Shoulders � =0.00331 � =0.602Total

Injury

Total

Injury

Since roadway width (and shoulder width) are constant for each curve, these cannot be

responsible for the non-linear form of the best fitting curve. It follows that the declining accident rate

noted in Figure 3 is probably not due to the association of roadway width with ADT. Also

noteworthy is that in Figures 5 and 6, the contribution of the wider roadway to safety is not clear cut.

Thus, e.g., if one is to believe the model equation, then, in North Carolina, 11 ft and 12 ft lane roads

have very nearly the same injury accident frequency for all ADTs. Surprisingly, for ADT>5000, roads

with 12 ft lanes in North Carolina have somewhat more accidents than roads with 11 ft lanes. The

opposite seems to hold In Washington.


The point of this section was to show that much of the early research, which relies on single-

variable tables and graphs of the relationship between the accident rate and roadway width (e.g.,

Figure 1) suffers from serious confounding. Roads with little traffic come with several traits that

affect their safety, only one of the traits is the lane width. The accident frequency on such roads

reflects the effect of all these traits, not only of lane width. When lane width is made to be the sole

independent variable, as in Figure 1, the apparent relationship is a mixture of the joint effect of all the

other traits that tend to go with narrow lanes - narrow shoulders, sharp curves, unforgiving roadside,

etc. In conclusion:

� the tendency of accident rates to decline as lane width increases is not an indication of a

cause-effect relationship. The accident rate usually declines as ADT increases for a variety of

reasons. Narrow roads and lanes tend to be associated with low traffic and therefore with

higher accident rates.

� Since ADT is a major determinant of road features, the safety effect of a feature can be

investigated only when ADT is held constant or its influence is otherwise accounted for.

3. Empirical evidence.

In this section I will attempt to review what empirical evidence exists in the literature. I will

do so without dwelling on those items which suffer from the one-variable-only confounding discussed

in section 2.

1953. Recognizing the complex interactions between the many variables, Raff (1953) examined

accident rates on two-lane tangents by volume of traffic, shoulder width and pavement width. He

concludes that “neither pavement width nor shoulder width nor any combination of them has a

determinable effect on the accident rates on two-lane tangents.”(p.29). This conclusion may reflect

the poor quality of the data available for analysis and the fact that data has been pooled for 15 states

with differing reporting requirements and reporting standards..

1954. One of the classical studies is by Belmont. Although the focus of the inquiry was on shoulder

width, the paper contains valuable evidence about the effect of lane width. The data pertains to rural

two-lane tangents, without structures or intersections, predominantly straight and level and with a

55 mph speed limit. Since techniques of analysis have improved in the interim, it seemed worthwhile

to embark on a re-analysis. The model accidents/mile-year=0.0006ADT was obtained by Poisson1.003

0

0.5

1

1.5

2

Rat

io +

/- o

ne s

tand

ard

devi

atio

n

15 20 25 30 Pavement width [ft]

Least squares fit


Figure 7

regression. Using this model, the ratio (accident count)/(predicted accidents) has been computed for

various pavement widths and is shown by squares in Figure 7. It var ies mildly in the range of practical

lane width.

Including pavement width in the Poisson

regression I find:

a c c i d e n t s / ( m i l e - y e a r ) =

0.0019×ADT ×(1-34.04/PW1 . 0 2 8

+383.4/PW )2

in which PW is pavement width in ft. This

leads to the accident modification factors

(AMFs) in Table 2

Table 2. AMFs based on Belmont’s data.

Pavement Width in ft 18 20 22 24 26 28 30

AMF 1.21 1.05 1.00 1.01 1.06 1.13 1.21

The merit of Belmont’s data is that they pertain to fairly homogeneous road sections (straight, level,

no structures) and exclude intersection accidents.

1955. The earliest before-after study of pavement widening is by Cope. The data are for 22 pavement

widening projects 244.3 miles long. In most cases, widening was from 18 to 22 feet. Accidents at

driveways, entrances and intersections were not included in the study. The dramatic reductions in

accident rates in Table 3 are reported.

Table 3. Before-After results by Cope

Accidents/MVM Number of projects Percent reduction in

Before accidents/MVM

<1.5 2 21.5

1.5-1.9 6 25.2

2.0-2.4 7 34.4

>2.5 7 46.6

0

0.5

1

1.5

AM

F

1 2 3 4 Accident rate (Before)

Sites which have a higher-than-average observed accident frequency in one time period2

are expected to return to their average accident frequency in the next time period. Thisspontaneous reduction in accident frequency is not a sign of safety improvement. If such aspontaneous return to what is normal is claimed to be a safety improvement, a ‘regression-to-mean’ bias is said to exist.


Figure 8

The statewide average rate for this type of accident was 2.0. Thus one may suspect that a part of the

increase in effect with accident rate is a reflection of regression to the mean . That is, sections that4

during the before period had unusually high accident rates may have been selected for widening and

therefore part of the apparent improvement is just a return to the true mean for these sections. It is

now impossible to know whether this conjecture is true. I re-analysed the data in the form presented

below. Each square represents one project. The abscissa is the accident modification factor obtained

in a project. In project ‘A’ accidents increased by some 20% (AMF=1.2) while in project ‘B’ they

decreased about 65% (AMF=0.35).

There is indeed an indication that when

the before accident rate was unusually

high the apparent reduction is very

large. Ascribing this to the regression-

to-mean bias, the unbiased effect

seems to be indicated by the dashed

line. Thus, for a widening from 18 to

22 ft the AMF�0.7. This is equivalent

to a 8% reduction per foot of lane

widening up to 22 ft. This finding is

consistent with Belmont’s except that

the effect is somewhat larger.

1957. The next data analysis is by Perkins who, whilst interested mainly in the safety effect of

shoulder width, also provided and analysed data on accidents, lane width and ADT for two- lane

rural highways in Connecticut. His conclusion was that:

“ there is no definite relationship of accident rates to shoulder width. The accident

rates vary . . . and do not in any case follow a consistent trend. The same is true of the

relationship of the accident rate to pavement width.”

0

1

2

Acc

iden

ts/M

VM

9 10 11 12Lane width [ft]


Figure 9

I re-analysed this data set as well and came to the same conclusion. That is, after accounting for the

effect of ADT, the ups and downs of the accident frequency are haphazard and there is no apparent

relationship to either pavement or shoulder width. This may reflect poor data quality or the absence

of a relationship. Note that, unlike Belmont, Perkins did not take care to eliminate intersection

accidents from the data, nor to control for horizontal curvature, presence of structures etc. It is

therefore not surprising that no relationship with lane of shoulder width can be discerned.

1959. In this study Head extended his Oregon inquiries about the effect of shoulders to ‘urban

extensions of the highway system’. Of a total of 466 urban highway sections, 426 (186.4 miles)

permitted parallel parking and these were used as data. Accident records were for two years.

When examining correlation with accident rate, Head finds that “pavement width showed positive

correlations with accident rates. However, ....was normally fairly low. . . .The effective lane width

was generally positively correlated, howevr, there were frequent negative correlations ...”

The regressions were linear. Separate equations were estimated 2-lane and 4-lane in several ADT

categories and settings (urban, suburban, corporate, business, residential, mixed,...). Pavement width

(PA) has a positive regression coefficient in 11 equations, negative in 8 and is not included in 12. It

seems that if lane width has a n effect on safety, it has not been clarified by this work.

1970. Figure 9 is based on data in Dart and Mann. Since this is a one-variable-at-a-time

presentation, it is of little interest except for the slight increase in accident rate from 11 ft to 12 ft

lanes. Such an increase is difficult to reconcile with the argument in Section 2 and the shape of the

ADT vs. accident frequency relationship in Figure 3. That is, if the accident rate is declining as ADT

increases, and if roads with more traffic tend to have wider lanes, then one should see in a figure such

as 9 a monotonous decline in accident rate. Since

we see an increase at the right tail, this may be an

indication of a dis-benefit that goes with lanes

wider than 11 ft. This too is consistent with

Belmont’s findings and lends them more credibility.

Recognizing the multivariate nature of the

problem, Dart and Mann (1970) estimate a

regression model for accident rates. This being an

early attempt, the model suffers from many


shortcomings and the results can hardly be trusted. (Thus, e.g., the average accident rate is estimated

as 0.29 accidents/MVM when it should be around 2). In any case, the model implies that an increase

of lane width by 1 ft is associated with a decrease of the accident rate by 3.5%.

1973, 1975. Silyanov and Babkov show graphs similar to Figure 1 and Figure 8 on which they

compile data by many researchers from various countries (USSR, UK, Sweden, Germany, Hungary,

USA). They all form a band as in Figure 1, showing declining accident rates with roadway width. As

far as I can tell, the method of all these studies is the same as that discussed in Section 2. That is,

accident rates were computed for road sections having the same roadway width. The association

between ADT and roadways width has not been accounted for. This means that the results are

confounded and one cannot say what is due to roadway width and what is due to all the other factors

which vary with ADT. The noted decline in rates may be caused by the many factors that are

associated with traffic flow. Drawing a bold curve through the diverse results by many researchers,

Babkov (1975) gives the numbers in Table 4.

Table 4. Relative Accident Rate by Babkov (1975).

Roadway width [m] 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 9.0

Roadway width [ft] 15.0 16.7 18.3 20.0 21.7 23.3 25.0 26.7 30.0

Relative Accident rate 2.2 1.7 1.4 1.3 1.1 1.05 1.0 0.9 0.8

1978. An elaborate regression analysis was performed by Roy Jorgensen Associates . Based on data

from Maryland, New York and Washington, initially 36 linear regression models were fitted in 4 ADT

categories × 3 categories of horizontal curvature × 3 categories of shoulder type. Terrain, pavement

width and shoulder width were the independent variables. The model was found to explain little. The

authors comments that: “. . . this approach was not adequate. Changes in sign of the regression

coefficients were commonplace, indicating that the model was not explaining the true physical

relationship between accident rate and highway geometrics, if one existed. These findings indicate that

straight lines do not effectively explain how accident rates vary . . . (with) shoulder and pavemen t

width or across ADT levels ” p.13. Thus, abandoning the linear regression model fitting idea, th e

authors resolved to estimate a multiplicative model for accident modification factors. The result is

shown in Table 5.


Table 5. Accident Modification Factors for rural two-lane highways based on

combined and smoothed accident rates in Maryland and Washington (p.18).

Pavement width [ft]Shoulder <18 19-20 21-22 >23

width [ft]

1-2 1.76 1.55 1.41 1.57

3-4 1.44 1.27 1.15 1.29

5-6 1.27 1.13 1.02 1.14

7-8 1.14 1.00 0.91 1.02

>9 1.11 0.99 0.90 1.00

The authors note that the increase from the 21-22 ft category to the >23 ft group is “inconsistent with

the expectation” but also emphasize that “it is consistent with the research findings”(p. 20.) In spite

of this, they decided to join the two rightmost columns into one width categories saying that “This

has the effect of conservatism in estimating the geometric effects on safety”. McLean ,( p.192)

questions this decision and maintains that while the increase in accident rates from the 21-22 ft

category to the >23 ft category “. . . may have been anomalous in terms of conventional engineering

expectations, they are consistent with the general hypothesis of an interaction between driver

behaviour and geometric standard.”. The questionable reasoning in Roy Jorgensen and Associates

(1978) resulted in the oft-quoted AMFs from their Table 13 and reproduced in row 2 of Table 6 as

AMF (modified). Were the unmodified data used the result would be that in Table 7.

Table 6. Modified AMFs

Pavement Width in feet 18 or less 20 22 24

AMF (modified) 1.18 1.04 1.00 1.00

Table 7. Unmodified AMFs

Pavement Width in feet 18 or less 19-20 21-22 >23

AMF (unmodified) 1.25 1.10 1.00 1.11

Note that the unmodified results are a more pronounced version of the re-analysis of Belmont’s data

and indicate that for rural two-lane roads, pavement widening beyond a certain point (21 to 22 ft) is

detrimental to safety.

0

0.5

1

1.5

2

Run

-Off-

The

-Roa

d A

ccid

ents

/MV

M

0 2000 4000 6000 8000 10000 Average ADT

18-20 ft.

22-24 ft.

0

0.5

1

1.5

2

Opp

osite

Dire

ctio

n A

ccid

ents

/MV

M

0 2000 4000 6000 8000 10000 Average ADT

18-20

22-24ft.

14-16ft.


Figure 10 Figure 11

1980, 1981. Zegeer et al. merged data for about 17,000 accidents (in Kentucky) with highway

section data for 25,670 km of road. The authors focussed on run-off-the-road and opposite-direction

accidents (apparently excluding rear-end, intersection and driveway related accidents as unrelated to

the issue of lane or shoulder width). The large majority of roads (70%) had no shoulders. The main

results are shown in Figures 10 and 11.

If one may assume that in the regions of overlapping ADT the compared road sections have similar

traits except for lane width, the results indicate that increased lane width is associated with very

substantive accident reductions in ‘ run-off-the road’ and ‘opposite direction’ accidents. The average

increase in run-off-the-road accidents is by a factor of 1.12 per foot decrease in pavement width.

Thus, 18' pavements may be expected to have 1.12 =1.57 time more run-off-the-road accidents than4

22' wide pavements. The average increase in ‘opposite-direction’ accidents is by a factor of 1.21 per

foot of decrease in pavement width. The pavement width groupings in this report do not allow one

to judge whether accident frequency increases as pavements width grows beyond 23 or 24 feet.

These results indicate a much larger effect of pavement width than earlier studies. However,

one must remember that earlier studies estimated the effect on all accidents whereas in this study the

effect on two accident types was estimated. The relevant proportions are shown below.

0

1

2

3

4

5

Acc

iden

ts/M

VM

7 8 9 10 11 12 Lane width [ft.]

No Shoulder

1'-3'

4'-6'

7'-9'

10'-12'


Figure 12

Table 8. Proportions of accidents by type.

Run-off-road 0.45

Opposite Direction 0.16

Other 0.39

Total 1.00

The authors note that: “Accident rates for other accidents generally increased as lane width increased,

indicating that the only accidents which would be expected to decrease with lane widening were the

run-off-the-road and opposite-direction accidents.” (1980, p.4). Of course, if one assumes that lane

widening reduces some accident types, one must be willing to accept that other accident types are

increased by lane widening (perhaps due to increased speed). It is the joint effect on all accident types

which ought to be of interest. Since this joint effect is n ot estimated here, one may only conclude that

it is lesser than what has been estimated in this study and may be in line with the results obtained in

the earlier studies.

The authors also show how accident rates vary with shoulder and lane width (Figure 12)

It is interesting to note that here, as on many previous occasions, there is an upturn in the accident

rate after a width of about 11 feet. Of course, as noted in section 1, not much credence should be

given to representations that do not account for the possible non-linearity in the effect of ADT on

accident frequency. Comparing accident rates in overlapping bands of ADT as in Figure 10 and 11


is much more convincing. ADT alone may not sufficiently account for differences in roadside hazard,

horizontal curvature, terrain etc.

1982. McBean studied the prevalence of selected geometric features at sites where an accident has

occurred and nearby sites subject to the same traffic and other influences. There were 197 site-pairs.

As is shown in Table 9 (his Table 13), 95 control sites but 121 accident sites had a carriageway width

wider than 6 m. Were carriageway width unrelated to accident occurrence, one would expect to find

both numbers to be approximately the same. Thus the finding is that accident sites tended to have a

wider carriageway

Table 9. Number of accident and control sites by carriageway width.

Accident sites

6 m or less wider than 6 m

Control 6 m or less 60 42

sites wider than 6 m 16 79

Thus, while 121 accident sites had a carriageway width wider than 6 m, only 95 control sites did.

This association persisted even when sites on curves were eliminated.

1983. Heimbach et al. use data for 57 sections of four-lane undivided urban highways with

intersections more than 2000 feet apart in eight urbanized areas in North Carolina. A total of 1936

accidents was used to develop multivariate linear regression models. In these, accident frequency is

related to ADT, intersection density, access density, lane width and alignment change magnitude. The

authors conclude that as lane width increases accidents decrease. While the regression equations are

complex, in an illustrative example the authors show that decreasing the total lane width from 48 ft.

to 40 ft. increases the number of accidents by a factor of 1.25. This is in line with the examination of

Heimbach’s results by McLean (1997) who estimates that there is a redu ction of 2%-2.5% in accident

rate for an increase of 0.25 m in lane width.

1986. Harwood assembled a data base intended to study the effect of cross section design on

multilane suburban highways. He finds that the accident rate depends on the proportion of truck

traffic, type of development, shoulder width, driveway and intersection density. He concludes that

the effect of ADT, lane width, left-turn demand and speed are not statistically significant. This does

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

acc.

/mile

-yea

r

18 20 22 24 26 28 pavement width

0<ADT<400

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

acc.

/mile

-yea

r


400<ADT<700

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

acc.

/mile

-yea

r


700<ADT<1000

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

acc.

/mile

-yea

r


1000<ADT<1500


not mean that the effect does not exist, only that with the available data and modelling strategy it

could not be identified.

1987 a. Griffin and Mak examined the benefits that could be achieved by widening rural two-lane

farm-to-market roads in Texas. The data pertains to 36,215 miles of such road in 1985. The bulk of

the data was for roads 18, 20, 22, 24, 26 and 28 ft. wide. Safety was measured in accidents (either

single-vehicle or multi-vehicle) per mile-year in four ADT categories (0-400, 400-700, 700-1000 and

1000-1500). The results are shown in Figures 12-19 in which the estimates (shown as squares) are

bracketed by one standard deviation (shown by triangles).

Figures 13-16. Single-Vehicle Accidents.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14 ac

c./m

ile-y

ear


0<ADT<400

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

acc.

/mile

-yea

r


400<ADT<700

0

0.1

0.2

0.3

0.4

0.5

0.6

acc.

/mile

-yea

r


1000<ADT<1500

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

acc.

/mile

-yea

r


700<ADT<1000


Figures 16-19. Multi-Vehicle Accidents.

The emerging picture is far from clear. The erratic jumps are explainable pa rtly by the imprecision that

is due to limitations of sample size. Attempts to fit a curve to the data met with limited success. On

the whole, the authors conclude that “Surface width has no demonstrable effect on multi-vehicle

accident rate on rural, two-lane farm to market roads with ADT’s up to 1,500" (which are about 33%

of the total) and that “surface widening can reduce single vehicle accident rate ...” (single vehicle

accidents are about 67% of the total in this data).

1987.b. Zegeer and Deacon use data from Kentucky (Zegeer, Deen, et al. 1980) and from Ohio

(Foody & Long ) to provide the best estimate then available. After making a series of assumptions

they find:


(Number of Run-Off-Road + Opposite Direction Accidents)/MVM=4.1501×0,8907W

×0.9562 ×1.0026 ×0.9403 ×1.0040W S W×S P L×P

in which W is Lane width in feet, S shoulder width and P is the width of the stabilized component of

the shoulder.

Because of the several judgement-based ‘correction factors’ the equation is not the result of a curve

fit in the usual sense but a judgement-composite of several data sets and research results. The authors

state that: “Because of the many assumptions necessary . . ., this model is not considered to be a

precise representation of the effects of lane and shoulder conditions on accident rates . . .”.(p.20).

This agglomeration of assumptions was necessary in order to come up with some guidance for TRB

Special Report 214 ( Designing Safer Roads, 1987). In spite of the speculative nature of this model

and its lack of representation of the data on which it is based, it has had a substantive influence on

practice.

1987 c. Zegeer et al. (1987) conducted a major study intended to quantify the benefits of lane

widening, shoulder widening, sideslope flattening etc.. Accident, traffic, roadway and roadside data

was collected on 1,944 road sections covering 4,951 miles of two-lane roads in seven states and

62,676 accidents. The main product of this work is a number of multivariate models. The models of

interest here are:

Total Accidents/(mile-year)=0.0015×ADT ×0.8897 ×0.9403 ×0.9602 ×1.20.9711 W PA UP H

Relevant Accidents/(mile-year)=0.0019×ADT ×0.8786 ×0.9192 ×0.9316 × 0.8824 W PA UP

1.2365 ×0.8822 × 1.3221H T1 T2

in which

ADT is the average daily traffic, W is the lane width in feet, PA is the average width

of the paved shoulder in feet, UP is the average width of the unpaved shoulder, H is

the median or roadside hazard rating(1 to 7), T1 is 1 if the terrain is flat and 0

otherwise, T2 is 1 if the terrain is mountainous and 0 otherwise, and ‘relevant’ are

single-vehicle+opposite direction head-on+opposite direction sideswipe+same

direction sideswipe accidents.


If these relationship can be taken to represent cause and effect, then, changing lane width from

W1 to W2 means that the frequency accidents is expected to change be a factor of 0.8897 . ToW2-W1

illustrate, if on a stretch of road with 11 ft lanes ,10 accidents/year are expected, then with 12 foot

lanes 10×0.8897 =8.9 accidents/year would be expected. The same (11% reduction) would obtain1

by going from 12 to 13 foot lanes or by going from 13 to 14 foot lanes. Much of the eralier evidence

indicates that the benefit of lane widening tapers off as lanes get wider and perhaps at certain width

further increase is detrimental to safety. The contrary result in the equations above comes not from

the data, but from the choice of the model equation. The function used (constant ) can neverW

represent a relationship that becomes flat at a certain lane width and begins to turn up.

As a rule, multivariate models of this kind cannot be trusted to represent cause-and-effect.

They only reflect the various co-variations that are present in the data used. In this case, data have

been pooled from 7 states without accounting for differences between them. Yet major differences

have been found between their accident rates, perhaps due to differences in accident reporting. If the

states differ in some systematic way in their lane-width inventory, then what is attributed here to lane

width may in fact be a reflection of differences in accident reporting. Similarly, while lane width is

correlated with average curvature (r=-0.36) and driveway density (r=-0.296), these variables do not

show up in the equations. Thus, since wider lanes are associated with less curvy roads and fewer

driveways, the safety benefit attributed to wider lanes may be a reflection of less curvy roads or

fewer driveways.

1991. Goldstine analysed data from 25 projects on 152 miles of two-lane rural roads in New

Mexico. The thrust of the paper is to compare accident rates before and after roadway widening

based usually on 2 years of accident data before and after construction. The accident rate was found

to be markedly reduced. The possibility of regression to the mean has not been considered. It is

difficult to know how much of the reduction is due to lane widening, how much due to shoulder

paving and how much due to other improvements (many projects had changes in sideslopes and in

vertical curvature).

1990. Harwood examined whether narrower lanes on urban arterials affect safety adversely. Data

were available for 35 (27 miles) projects involving narrowing of lanes. All involved changes in cross-

section where lanes were added. Therefore, it was not possi ble to isolate the effect of narrower lanes

from changes such as the introduction of TWLTL or the removal of a median.

Where a two-lane undivided road was converted to a four-lane undivided road, there was a large

increase in accidents. But the increase was at intersections and driveways and had little to do with

1.8

2

2.2

2.4

2.6

2.8

3

Rel

ated

acc

iden

ts/M

VM

15 20 25 30 35 40 45 Width of lanes + shoulders [ft]


Figure 21

lane width. Where a five lane (with TWLTL) cross-section was made into a seven lane (with

TWLTL) cross-section both mid-block and intersection accident rates increase (20-30%). Where a

six-lane divided road was made into an eight-lane divided road accident rate increased at intersections

but not mid-block.

1994 a. Zegeer et al. examined the effect of lane and shoulder width on low volume roads carrying

fewer than 2000 vehicles per day. The ‘primary’ database entailed 4,137 miles of road in seven states.

An additional ‘cross-section data’ base from three states was used to validate the results.

As in Zegeer et al. (1980, 1981), only single-vehicle and opposite-direction accidents were

considered ‘relevant’. The authors state that: “Rates for other accident types were found not to be

significantly related to lane or shoulder width.” (p.163). This seems contrary to the earlier quote

(“Accident rates for other accidents generally increased as lane width increased, indicating that the

only accidents which would be expected to decrease with lane widening were the run-ff-the-road and

opposite-direction accidents.”1980, p.4). The ‘relevant’ accidents are 63% of the primary data base

and only 39% of the cross-section data base. If the remaining 37%-61% accidents tend to become

more numerous as lane width increases, they should not be omitted from the analysis.

Surprisingly, the method of analysis chosen was to examine how accident rates vary with lane

and shoulder width, without accounting for the possibility of non-linear effect of traffic flow (but

adjusting for roadside hazard, terrain, State, and driveway density). Thus, the results may be subject

to the confounding discussed in Section 2. That is, an undetermined part of the decline in accident

rate with lane+shoulder width may be

due to the non-linear relationship

between accident frequency and ADT.

The main results are shown in Figure

21 .


An attempt has been made to examine the surprisingly low accident rate for pavement width of 18

ft. or less by examining new data from Illinois, Minnesota and North Carolina as shown in Table 9.

Table 9. ‘Related’ Accidents/MVM.

Lane width [ft] 8 9 10 11 12

Illinois 3.57 1.13 2.03

Minnesota 2.32 0.85 1.03 0.67

North Carolina 1.95 1.94 1.73 1.69

One may expect that the true relationship between safety and lane width is smooth and gradual. Thus,

e.g., it would be most perplexing if in Illinois 9 ft lane width was associated with a third of the

accidents for 8 ft lanes but twice as many as for 10 ft. lanes. The accident rate jumps in Figure 21 and

in Table 9 make it difficult to distinguish between what is signal and what is noise. In general, jumps

of this kind may be an indication of:

a. Insufficient data, so that some estimates have a large standard error;

b. Poor data quality;

c. Presence of co-variation with other variable that are not represented in the analysis or

considered incorrectly.

If the general trend in Figure 21 is a reflection of the safety effect of lane+shoulder width, then one

might conclude that increasing this width by 1 foot is associated with a 1.5% decrease in the rate of

‘related’ accidents. This is much less than what has ben found in all earlier studies. The ‘related’

accidents (single vehicle and opposite direction) are 37%-61% of all accidents.

The author also have important data (Table 9) to shed light on the question whether lanes wider than

11 of 12 feet are safer.


Illinois Minnesota

11 12 11 12

Shoulders [ft.] 0-2 �3 0-2 �3 0-2 �3 0-2 �3

Related accidents/MVM 1.12 0.88 0.84 0.85 0.67 0.61 0.72 0.52

Table 9

The authors conclude that “little if any rea l accident benefit can be gained from increasing lane width

from 3.4 m (11 ft.) To 3.7 m (12 ft.) On low volume roads.”

1994 b. Urbanik discusses the experience gained in narrowing lanes and shoulders on urban

freeways. Based on work reported in (McCasland and Urbanik & Bonilla) they maintain that

following the implementation of 24 projects (in which capacity was increased by adding a new lane

at the expense of shoulder and lane width) “... most sites experienced decreased accident rates after

the projects were implemented ...”(p. 126). They also note that “narrowing of lanes to 11 feet (or

occasionally 10.5 feet) while maintaining shoulders did not change accident rates.” Based on the

review of several projects in California the authors note that: “. . . higher accident rates had not

materialized several years after lanes were narrowed and left shoulders were removed . . .”. They also

claim that “accident migration is not a problem on well designed projects”.

1995 a. Zegeer and Council review (briefly) what is known about the safety effects of cross-section

elements. No new data is analysed. On the effect of lane width the recommend to rely on the findings

by Zegeer et al. (1987).

1995 b. Hadi et al used four years of Florida crash data to estimate NB models for nine road classes.

The function e has been chosen to represent the influence of lane width. The following�×Lane width

values of $ were obtained.

0

100

200

300

Acc

./100

-MV

M A

ltere

d se

gmen

t

0 100 200 300 Acc./100-MVM Unalrered segment


Figure 22

Table 10. Regression coefficients for Lane Width and Pavement Width, Mid-block crashes.

Lane width Pavement width

Two-lane, rural -0.0857

Four-lane, rural, divided Not included Not included

Four and six-lane rural freeways Not included Not included

two-lane, urban, -0.355

four-lane, urban, undivided -0.1127

four-lane, urban, divided Not included Not included

six-lane, urban, divided Not included Not included

four-lane, urban, freeway -0.3909

six-lane, urban freeway -0.3504

Whether lane width was included amongst the regressor variables has been decided in the stepwise

regression by the Akaike information criterion. Thus, the non-inclusion of lane width in certain

models merely indicates that it was deemed statistically insufficiently important. The functional form

used forces the conclusion that crash frequency decreases with lane width no matter how wide the

lane. The implied reduction in crash frequence per foot increase in width (e.g., 39% on four-lane

urban freeways) seems quite excessive.

For some cases the authors give the lane width category that minimizes crashes. For two-lane

rural roads =4 m; four-lane, rural divided=3.0-3.7 m; two-lane, urban 3.7 m; four-lane urban,

undivided=4.0 m; four-lane, urban, divided= 3.0 m; four-lane, urban, freeway= 4.0 m.

1995 c. Curren examined the safety consequences of

increasing freeway capacity by use of shoulders and

narrow lanes. Accident data were for 3-3.5 year for

corridors I-95, I-395, I-5, I-90, I-85 and I-10 in Virginia,

Washington, Georgia and California. A total of 12795

accidents was used on 49.49 altered miles and 35.03

unaltered miles on the same corridor. The comparison

of accident rate for ‘altered’ and ‘unaltered’ segments is

shown in Figure 22.


Treated Comparison

Before 86 89

After 122 75

Table 10. Suburban Freeway

Treated Comparison

Before 257 206

After 287 183

Table 11. Urban Freeway

As is obvious, different freeways segments have very different accident rates Ranging from 80 to

280). The idea that one can learn something form comparing altered and unaltered segments rests

on the expectation that the compared segments (Altered and Unaltered) should have the same

accident rate. This expectation is not well founded.

One segment (I-5) provided the opportunity to do a Before/After comparison.

It seems that where shoulders and narrow lanes were used that accident rate increased by a factor of

1.68 (suburban freeway) and 1.26 (urban freeway).

1995 c Miaou used data from Utah for 11539 two-lane rural undivided road sections and 6680 single

vehicle accidents for eight years to estimate a multivariate model. The effect of lane width was not

statistically significant, most likely because 96.5% of the road sections had 12 ft. lanes.

1996 a. In a synthesis of Australian and international experience for rural roads McLean also relies

mainly on results by Zegeer et al (1987) as being “the most comprehensive of the studies and

(therefore) should be regarded as the benchmark against which others are compared”(p.9).

1996 b. Miaou used data on 596 two-lane rural road sections in Alabama, Michigan and Washington

to model the relationship between 4632 single vehicle accidents in 1980-84 and various geometric

and traffic traits. He finds that increasing lane width by one foot decreases the number of single

vehicle run-of-the-road accidents by 14%.

1996 c. Miaou used the data originally analysed by Zegeer et al. in 1987. Of the 1944 sections

originally used a subset of 1282 pure rural sections was selected. With 29,262 accidents. The

covariates used were: dummy variable for ‘State’ AADT/lane, Lane width, Shoulder width, roadside

recovery distance, horizontal curvature, terrain type, vertical grade, sideslope, intersections/mile,

driveways/mile, bridges/mile, RHR. The regression coefficient is $ =-0.078. Thus increasinglane width

lane width by ) ft. Has an AMF=e . -0.078�

1

1.2

1.4

1.6

Acc

iden

t Mod

ifica

tion

Fun

ctio

n

0 500 1000 1500 2000 2500 3000 Average Daily Traffic

Run-Off-road and Opposite Direction

1.50; 9 ft.

1.30; 10 ft.

1.15; 11 ft.

1.00; 12 ft.1.051.02 1.01


Figure 23

1997. In September 1997 a group of experts met in Washington for a few days with the purpose of

reaching consensus on the accident mod ification factors needed as input for the Interactive Highway

Safety Design Model (IHSDM). The results of this deliberation are shown in Figure 22. Thus, the

effect on single-vehicle and opposite direction accidents is minimal for ADT<400 and increases

linearly up to the limiting values shown. Because of the results obtained by Zegeer et al. (1994) the

panel thought that the widening of 9 ft lanes to 10 ft. is undesirable.

The assumption is that only run-off-road and opposite direction accidents are affected by lane

widening. The proportion of such accidents depends on ADT. If the information in Figure 22 is to

be applied to total accidents an appropriate correction needs to be applied. Thus, e.g., if half of the

accidents are of the run-off-road and opposite direction accidents type, the AMF in figure 22 needs

to be halved.

1998 a. Wang et al. examined the influence of cross-section elements on the safety of rural, multi-

lane, non-freeway roads using HSIS and photolog data from Minnesota. Although data about

number of lanes and road surface width were available, the estimated model does not include these

variables; presumably they did not reach a desired level of statistical significance.

1998 b. Stewart and Council fitted simple models to data from North Carolina and Washington for

rural two-lane and four-lane roads.

A. Two-lane roads NC.

When separate models were fitted to data with 22 and 24 ft pavement width (keeping shoulder width

at 6 ft.), then for ADT<�5000 the wider pavement was safer and for ADT>5000 the narrower


Driveways/km

To

0.1 5 10 15 20 25 30

From

0.1 1.00 1.05 1.11 1.16 1.23 1.29 1.36 5 0.95 1.00 1.05 1.11 1.17 1.23 1.29

10 0.90 0.95 1.00 1.05 1.11 1.17 1.23 15 0.86 0.90 0.95 1.00 1.05 1.11 1.17 20 0.82 0.86 0.90 0.95 1.00 1.05 1.11 25 0.78 0.82 0.86 0.90 0.95 1.00 1.05 30 0.74 0.77 0.82 0.86 0.90 0.95 1.00

Table 12. AMFs for ADT=10,000

pavement was safer. However, up to ADT<8000, there was little difference between the two. When

a single model was fitted to data (surface width 20-24 ft., shoulder width 4-10ft) $ =-0.037surface width

and $ =-0.037 were obtained. For injury accidents $ =-0.042 and $ =-0.048shoulder width surface width shoulder width

were obtained.

B. Four-lane rural, non-freeways NC.

For one class of undivided highways surface width were in the 44-52 ft. Range with shoulders from

4-12 ft. The regression parameters for surface and shoulder width were not statistically significant.

For another class (with curbs) surface wifth ranged from 60 to 68 ft. No comment about the effect

of surface width has been made.

C. Two-lane roads WA.

When separate models were fitted to data with 22 and 24 ft. pavement width (keeping shoulders at

6 ft.) There is again a crossover as in NC, but in the opposite direction. The two curves are quite

dissimilar for higher ADT values. When a single model was fitted to the data $ was 0.000.suface width

(The effect of shoulder width was now suspiciously large).

D. Four-lane rural, Non-freeway, WA.

For undivided roads neither shoulder nor surface width were found to have statistically significant

regression parameters. For divided roads there was no variation in surface width.

1998c. Vogt and Bared used data from Minnesota (704.5 miles, 1694 accidents in three years) and

Washington (535 miles,1706 accidents in three years) to estimate models for rural two-lane roads.

The models account for ADT, roadway width, Roadside Hazard rating, driveway density , average

horizontal curvature, average vertical curvature and average grade. The resulting regression


Two-lane Four-lane, Undivided Four-lane, Divided

North Carolina 4900 325 insufficient data

Washington 1796 67 insufficient data

Minnesota 4370 414 insufficient data

California 3747 279 110

Table 13. Miles of road in sample.

�0 �1 �2 �3

North Carolina 2-lane -2.9915 0.6725 -0.123 -0.1506

4-lane,divided -4.6914 0.7615 -0.2877

Washington 2-lane -6.2152 0.9669 -0.4541

4-lane,divided -4.5387 0.6355

Minnesota 2-lane -8.1823 1.1758 -0.2949

4-lane,divided -7.2548 1.0644 -0.2339

California 2-lane -3.0188 0.9048 -0.3419 -0.4167

4-lane,divided -8.9871 1.0707

4-lane,undivided

-8.7176 1.1213

Table 14. Parameter estimates.

coefficients for lane width when data from bath states was combined was $ =-0.085. Thus anlane width

increase of lane with by ) ft. Gives an AMF of e- .0.085�

1999. Council and Stewart developed models to predict crashes/km-year for typical two-lane, four-

lane undivided and four-lane divided roads. Data from California, Washington, Michigan and North

Carolina served for analysis. The available miles of road by state and road type is in Table 13.

Only non-intersection and non-intersection-related crashes were used in the comparison. The model

was of the form: crashes/km=Length×e ×ADT ×e ×e . The parameter�0 �1 �2×shoulder width �3×Surface width

estimates are in Table 14.

The parameter for surface width was statistically significant only for two-lane roads and only in two

of the four states. In North Carolina widening the surface width by 1 m reduces crashes by 15%, in

California by 41%.


4. Summary

1. A great deal of empirical evidence has been accumulated over several decades. The bulk of

it pertains to two-lane rural roads. Little is known about the effect of lane width on multilane roads

or urban roads.

2. When road sections differ in lane width they tend to differ also in other important respects.

This makes the isolation of the safety effect of lane width difficult.

3. In spite of this difficulty, there is a great deal of congruence between the results. Thus, the

AMFs obtained by Belmont (1954), Cope (1955), Roy Jorgensen (1978), Zegeer et al. (1987) and

Miaou (1996) are very similar when brought to the common denominator of ‘all accidents’.

4. There is, however, one issue on which opinions differ. Most early researchers found that the

safety benefit of lane widening bottoms out somewhere between 11 ft. and 12 ft. Further widening

seemed to be to the detriment of safety. Later researchers, using perhaps better data and methods of

analysis, unfortunately choose to use in their models a functional form that can never reach a

‘bottom’. Nor is there any evidence in their work that before choosing this functional form they

examined whether their data indicated an increase for wide lanes. For this reason, in my opinion, the

weight of the extant empirical evidence indicates that there is little safety benefit to be obtained from

widening lanes beyond 11 ft and that widening beyond 12 ft may be to the detriment of safety.

5. There is some empirical evidence about the safety effect of reducing lane width on urban

arterials and freeways when the aim is to add a lane to increase capacity. This evidence is difficult to

interpret in terms of the safety effect of lane width because when a lane is added (even when no other

changes are made) the flow/lane is significantly changed.

References.

(1965). Research on road traffic. Her Majesty's Stationary Office, London.

(1987). Relationship between safety and key highway features. 6. State of the Art Report, Transportation research

Board, Washington D.C.

Babkov, V. F. (1975). Road conditions and traffic safety. Mir, Moscow.

Belmont.D.M.,(1954), Effect of shoulder width on accidents on two-lane tangents. Highway Research Bulletin. 91,

29-32.

Council, F. and Stewart, J. R. (2000) Safety effects of the conversion of two-lane rural to four-lane rural roadways based

on cross-sectional models. Transportation Research Board Annual Meeting.

Cope, A. J.,(1955), Traffic accident experience - before and after pavement widening. Traffic Engineering. 114-115


Curren, J. E.,(1995). Use of shoulders and narrow lanes to increase freeway capacity. 369. National Cooperative

Highway Research Program, AASHTO, Washington, D.C.

Dart, K. O. and Mann, L., Jr.,(1970), Relationship of rural highway geometry to accident rates in Louisiana. Highway

Research Record. 313, 1-15

Foody, T. J. and Long, M. D.,(1974). The identification of relationships between safety and roadway obstructions.

Ohio-DOT-06-74. Ohio Department of Transportation, Columbus, Ohio.

Goldstine, R.,(1991), Influence of road width on accident rates by traffic volume. Transportation Research

Record. 1318, 64-69

Griffin, L. I. III and Mak, K. K.,(1987). The Benefits to be achieved from widening rural, two-lane farm-to-market

roads in Texas. 67- th Annual Meeting of the Transportation Research Board, 1988. The Texas State Department of

Highways and Public Transportation, Texas.

Hadi, M.A., Aruldhas, J., Chow, L-F, Wattleworth, J.A., (1995) Estimating safety effects of cross-section design for

various highway types using negative binomial regression, Transportation Research Record 1500, 169-177,

Washington, D.C.

Harwood, D. W.,(1986). Multilane design alternatives for improving suburban highways. 282. National Cooperative

Highway Research Program, Washington, D.C.

Harwood, D. W.,(1990). Effective utilization of street width on urban arterials. 330. National Cooperative Highway

research Program, Washington, D.C.

Head, J. A.,(1959), Predicting traffioc accidents from roadway elements on urban extensions of state highways.

Highway Research Board Bulletin. 208, 45-63

Heimbach, C. L., Cribbins, P. A., and Chang, M-S,(1983), Some partial consequences of reduced traffic lane widths

on urban arterials. Transportation Research Record. 923, 69-72

McBean, P. A.,(1982). The influence of road geometry at a sample of accident sites. 1053. TRRL Laboratory Report,

Transport and Road Research Laboratory, Crowthorne, UK.

McCasland, W. R.,(1980). Freeway modifications to increase traffic flow. FHWA-TS-80-203. FHWA,

McLean, J.,(1996). Review of accidents and rural cross section elements including roadsides. ARR 297. ARR, ARRB

Transport research Ltd., Victoria, Australia.

McLean, J.,(1997). Review of accident and urban arterial cross-section treatments. 309. ARR, ARRB Transport

Research,

McLean, J. R. (1980) The safety implications of geometric standards. Canberra. Unpublished Work.

Miaou, S-P, (1996). Measuring the goodness-of-fit of accident prediction models. U.S. DOT, Federal Highway

Administration, FHWA-RD-96-040.

Miaou, S-P. (1996) Progress report. ORNL's modelling of two-lane rural road data. 1996. Oak Ridge National

Laboratory. Personal Communication

Perkins, E. T., (1957), Relationship of accident rate to highway shoulder width. Highway Research Bulletin. 151,

13-14.

Raff, M. S., (1953), Interstate highway-accident study. Highway Research Bulletin. 74, 18-43.

Roy Jorgensen Associates, Inc.,(1978). Cost and safety effectiveness of highway design elements. National Cooperative

Highway Research Program Report 197, Washington D.C.

Silyanov, V. V.,(1973), Comparison of the pattern of accident rates on roads of different countries. Traffic Engineering

and Control. 14(9) 432-435.


Slatterly, G. T. and Cleveland, (1969) D. E. Traffic control & roadway elements - their relationship to highway

safety/revise. Traffic volume. Highway Users federation for Safety and Mobility, Washington, D.C.

Stewart, D and Council, F. (1998)To smooth or not to smooth, that is the question. An analysis of accidents on rural

NC two-lane and four-lane roads. Personal communication.

Urbanik T, II,(1994), Factors affecting selection of lane width and shoulder width on urban freeways. Transportation

Research Record. 1445, 125-129.

Urbanik, T., II and Bonilla, C. R.,(1987). Safety and operational evaluation of shoulders on urban freeways.

FHWA/TX-87/32.

Vogt, A. and Bared, J.,(1998), Accident models for two-lane rural segments and intersections. Transportation

Research Record. 1635, 18-29

Wang, J., Hughes, W. E., and Stewart, R.,(1998). Safety effects of cross-section desing on rural multi-lane highways.

FHWA-RD-98-071.

Zegeer, C. V. and Council, F. M.,(1995), Safety relationships associated with cross-sectional roadway elements.

Transportation Research Record. 1512, 29-35.

Zegeer, C. V., Deen, R. C., and Mayes, J. G.,(1980). Effect of lane width and shoulder widths on accident reduction

on rural, two-lane roads. Research Report for the Kentucky Department of Transportation.

Zegeer, C. V., Deen, R. C., and Mayes, J. G.,(1981), Effect of lane width and shoulder widths on accident reduction

on rural, two-lane roads. Transportation Research Record. 806, 33-43.

Zegeer, C. V., Reinfurt, D., Hummer, J., Herf, L., and Hunter, W.,(1987). Safety effects of cross-section design for two-

lane roads. FHWA-RD-87/008. Federal Highway Administration.

Zegeer, C. V., Stewart, R., Council, F., and Neuman, T.,(1994 a), Accident relationships of roadway width on low-

volume roads. Transportation Research Record. 1445, 160-168.

Zegeer, C. V., Stewart, R., Council, F., and Neuman, T.,(1994 b), Roadway widths for low-traffic-volume roads.

National Cooperative Highway research Program Report 362, Washington D.C.

Zegeer, C. V. and Council, F. M.,(1995), Safety relationships associated with cross-sectional roadway elements.

Transportation Research Record. 1512, 29-35

Zegeer, C.V. and Deacon, J.A., Effect of lane width, shoulder width, and shoulder type on highway safety. In:

Relationship between safety and key highway features. State of the Art Report 6, Transportation Research Board,

Washington, D.C., 1987.

this is an unedited draft reflecting my personal opinions ... ·...

Documents