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A CONTROL OR BRAKING PROBLEMSlipperiness Contributes Particularly to

Oncoming Vehicle AccidentsCausing 25% of Fatali ies (Rear End

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Ski ding Accidents and Their Avoidancewith Different CarsLennart Strandberg

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A CONTROL OR BRAKING PROBLEMSlipperiness Contributes Particularly to

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Ski ding Accidents and Their Avoidancewith Different CarsLennart Strandberg

on Experimental Safety Vehicles, Gothenburg, Sweden

May 29 - June 1, 1989, pp 825-828

Reprint from

Samhall Khntland Granska, Llnkoplng 1991

Skidding Accidents and Their Avoidance with Different Cars

Lennart Strandberg,Swedish Road and Traffic Research Institute,

VTI

Abstract

In Swedish statistics from 1985 and 1986, the percentage

of oncoming vehicle accidents is four times greater on

snowy/icy roads (16%) than on dry clear roads (4%). The

percentage rear end accidents is the same (6%) for both

road conditions. Also the fatality numbers (364 people in

oncoming vehicle versus 26 in rear end accidents)

indicate that more emphasis should be put on instability and

skidding problems (typical for oncoming vehicle accidents)

than on braking distances and steering-while-braking

problems (rear end accidents).

Vehicle dynamics show that the cornering performance

of the rear wheels must be superior to that of the front

wheels, if a car is to be stable at all speeds. This is illustrated

by some hydroplaning fatality cases, where the tyre pattern

was much deeper at the front than at the rear. Since traction

or braking forces reduce the side force, skid recovery will be

more difficult in a front wheel driven car, if the driver

depresses the clutch pedal before countersteering.

Experiments with experienced drivers offered an

explanation to why such a behaviour still is recommended

in many driver training courses.

In uence on stability from vehicle design and equipment

is being even more pronounced with antilock (ABS-)

brakes, 4 wheel steering, cruise control, etc. This must be

considered in research on vehicle black spots in accidents,

in vehicle inspection and testing, and in driver training

programs.

Focus on Safety Relevant Properties inVehicle Technology DevelopmentAn increasing number of investigators claim that

improvements of crash avoidance properties in cars do not

increase the net driver-vehicle safety. For instance, the

distinct improvements with antilock (ABS-) brakes in

experimental driving tests found by Rompe et al (1987)

were contradicted by accident records from real traffic

according to Aschenbrenner et al (1988).

It has also been concluded that training of drivers on skid

pads and in advanced evasive manoeuvers may increase the

accident risk in real traffic. Glad (1988) found that drivers

with skidpad training were involved in more accidents per

mileage unit than comparable drivers without such training.

However, controllability (steering and braking)

performance had been emphasized with a disregard for the

directional stability properties both in the tests by Rompe et

al (1987) and in the skidpad training evaluated by Glad

(1988). This may explain the paradoxical findings by Glad

(1988) and by Aschenbrenner et al (1988), since there are

evidence from Swedish accidents and driving experiments

that poor stability may contribute to a much greater number

of serious accidents than what poor controllability does.

Therefore, the question on stability versus controllability

may be decisive both of safety and of public demands on

vehicle technology developments in the future.

In uence on stability from vehicle design and equipment

825

is being even more pronounced with ABS brakes, with

front/rear or four wheel drive, with cruise control, with four

wheel steering, with (prohibitions on) studded tyres, with

increased differences in tyre adhesion between wet and dry

conditions, etc. This must be considered in research on

vehicle black spots in accidents, in vehicle inspection and

testing, and in driver training programs.

Directional Stability VersusControllability in Accident Statistics

In official statistics of Sweden from 1985 and 1986, the

percentage of oncoming vehicle accidents with personal

injury is four times greater on snowy/icy roads (16%) than

on dry clear roads (4%), SCB (1986) and SCB (1987). The

percentage rear end accidents is the same (6%) for both

road conditions. When considering the Swedish accident

numbers from 1985 and 1986 to be representative samples

of these accident types, Strandberg (1988) found that their

difference in road slipperiness sensitivity was significant on

the 0.001 level (chi square 173 and 253 respectively).

Perhaps it is contrary to common belief, but slippery road

conditions are much more overrepresented in oncoming

than in rear end accidents. See figure 1. This indicates

that more emphasis should be put on instability and

skidding problems (typical for oncoming vehicle accidents)

than on braking distances and steering while-braking

problems (rear end accidents).

REAR END:

1 1 24 accidentsONCOMING:

1 044 accidents

Ice

Snow

Figure 1. Distribution of road surface conditions in two typesof police reported accidents with personal injury, Sweden 1986.Data from SCB (1987).

Also the fatality numbers in multiple vehicle crashes are

dominated by oncoming accidents. According to

Swedish statistics from 1985 and 1986, oncoming

vehicle accidents killed 364 people, while 26 persons died

upon rear end accidents. Figure 2 gives the division of

fatalities during 1986 between different types of accidents

between motor vehicles.

Superior Front Wheel CorneringPerformance Threatens Stability

A vehicle is stable if the resultant of external forces

applies to the rear of the centre of gravity (figure 3). When

aerodynamic forces are negligible, this condition is fulfilled

in a car at any speed (and without demands on compensating

826

steering), if the cornering performance (or cornering

stiffness coefficient as defined by SAE, 1975) of the front

wheels is inferior to that of the rear wheels. See Strandberg

(1983) and Strandberg et al (1983) for mathematical details.

331 killed people 1 986in 256 accidents between motor vehicles Others

Crossing

Exltlng

Overtaklng

Rear End

Oncomlng

0 20 40 60 80 100 120 140 160 180

Nunber of killed people

Figure 2. Number of killed persons in different types ofaccidents between motor vehicles, Sweden 1986. Data fromSCB (1987).

Since lateral tyre forces are reduced by longitudinal ones,

stability may be reinforced also if the front wheels are

overpowered or overbraked compared to the rear wheels.

Figure 3. External forces and their resultant acting on a vehicle(air- or waterborne ship above, and car below). Stability andspontaneous return to original direction, since the resultantforce applies to the rear of centre of gravity. Sequence of eventsand direction of motion from left to right.

Superior front wheel cornering performance, on the other

hand, may initialize spontaneous skidding and spin-outs.

That has been confirmed in a considerable number of

accident case descriptions. A pilot investigation of some

hydroplaning fatality cases, revealed that the tyre tread

pattern of the skidding and outspinning cars was much

deeper at the front than at the rear (table 1).

In the year of 1986, 327 car drivers and 155 passengers

were killed in Swedish traffic accidents, that is 9 car

occupant fatalities per week. According to table 1, rear

wheel skids and poor rear tyres may contribute about a

quarter of this average during certain time periods.

However, a more scientific and systematic approach is

needed before any general conclusions can be drawn on the

relative in uence from the front tyres compared to the rear

ones. Therefore, a case-control investigation is being

planned to study if poor rear tyres are overrepresented in

accidents on slippery road surfaces. See the last paragraph

of this paper.

Table 1. A sample of car occupant fatalities reported as hydroplaning accidents, where it has been attempted to assess the tyre treadpattern depth. Cases (2 8) discovered in Swedish newspapers from four summer weeks in 1988, and an additional accident (Acc. ref.no. 1) from 1987, the documents of which were investigated on request from a local Court.

TYRE PATTERN (um)

Rear Front Front Comments

Right Left Right

ACC. No. of DEVIATING CARRef. Date Killed Skid Wheels RearNo. People at Driven Left

1 July 25, 1987 3 Rear Front 2?2 July 25, 1988 1 Rear Rear 0-13 July 29, 1988 1 Rear Front 0 14 July 29, 1988 (1) Front Rear 2.5

5 July 30, 1988 1 Rear Rear Tyres6 Aug. 4, 1988 3 Rear Rear 0.5 1

7 Aug. 15, 1988 1 Rear Rear 2

8 Aug. 20, 1988 2 Rear Rear 1

No.2 8: 10No 2 8 but no.4: 9

No.1 8 but no.4: 12

< 1.6 New New BIN 794. Rear tyre photos only.Burned > 6 > 6 KY" 965. Car on fire.2 4 4 nm=99L

2.5 3 3 BZ.) 434. Plow out (Front skid).burned & destroyed Km 343. l linibus skidding.1 4 5 4 5 EBK 322.2 7 7 NZD 964. Single vehicle acc.1 3 4 3 4 TSUBB54.

Hydroplaning fatalities in 4 weeks discovered in this survey.Rear wheel skid fatalities in 4 weeks discovered in survey.Rear wheel skid fatalities. Total number discovered in survey.

Stability and Controllability inDifferent Situations and Cars

Directional (yaw) stability is often reduced when

controllability is improved. Therefore, some measures on

the car may deteriorate safety, though they are based on

many individuals experience from normal driving and

though they aim at increasing the driver s control over the

vehicle.

Also contributing to the contradiction is that skilled

drivers are able to better control the motion of the car by

forcing it into an unstable state. For instance, in competition

driving or the like it may be advantageous to turn the car

quickly and precisely. Then the throttle may be used to spin

the wheels and the hand controlled (parking) brake to lock

them up.

This was demonstrated in a television program (Billing,

1988), where the VTI test driver kept the car spinning in

about nine revolutions while its speed decreased from 70

km/h to standstill. The car was front wheel driven and the

hand brake acted on the rear wheels.

In the same broadcast it was made clear that severe yaw

motions, being extremely dangerous in real traffic, may

occur unintentionally and suddenly if the rear wheels are

overpowered or overbraked. Such cases may well be

initialized on icy roads by the automatic cruise control

devices, though they are considered contributing to safety

by helping the driver to keep the speed limits.

It has also been observed (Strandberg, 1988) that rear

skid recovering may be simplified by braking in certain

ABS-equipped cars. Since overbraking of the front wheels

reduces their side forces, the need of precise countersteering

becomes less pronounced.

Front Wheel Drive and Studded Tyres

Common driving experience indicates that severe yaw

and rear wheel skids are unlikely to occur in front wheel

driven cars. However, irrespecitve of front or rear drive,

Strandberg (1988) found spin outs more frequent than plow

outs in double lane change tests on ice with ordinary stud

ded tyres. Since then it has been observed that the stud

protrusion tends to increase at the driven wheels and to

decrease at the other ones, particularly in front wheel driven

cars. Therefore, front wheel driven cars with studded tyres

may be more susceptible to unexpected loss of stability on

winter roads.

Stud protrusion was measured by the VTI during the

winter 1988/89 on a sample of 200 cars in Sweden. To

achieve results representative for the Swedish tyre popula

tion, police officers selected the cars randomly from the

normal traffic ow on suitable roads. The average stud

protrusion was greater at the driven wheels. The difference

was most pronounced and statistically significant for front

wheel driven cars. See table 2, confirming that front wheel

driven cars with studded tyres tend to be less stable on ice,

when longitudinal tyre forces are negligible.

The corresponding hazard of front wheel drive is rein

forced by the fact that common education recommends

drivers (even of front driven cars) to depress the clutch

pedal before countersteering in a rear wheel skid. The rec-

ommendation may stem from successful tests with experi

enced drivers, who are unlikely to resist countersteering

until the skid is fully developed and to avoid any recovering

action before depressing the clutch pedal. See Strandberg

(1988) for further details.

Nevertheless, studded tyres on today s average car seem

to improve safety on ice and snow according to the prelimi-

nary results from a case-control (pilot) study carried out in

two Swedish police jurisdictions from November 1988 to

827

Table 2. Stud-protrusion-statistics. Data from Random selection of 200 cars in traffic on Swedish roads during winter 1988 89.(Report by Samuelsson in preparation.)

Driven Average Tyre Stud No. Variable Std. Std. Min. Max.wheels Protrusion of Mean Dev. Error value value Comments

(Variable Definition) Cars (mm) (mm) (mm) (mm) (mm)

Rear Rear Right 90 0.89 0.46 0.00 2.00

Rear Rear Left 90 0.90 0.47 0.00 1.90

Rear Front Right 86 0.84 0.44 0.00 1.70Rear Front Left 86 0.83 0.46 0.00 1.90

Front Rear Right 72 0.75 0.50 0.00 2.30

Front Rear Left 71 0.80 0.50 0.00 2.30

Front Front Right 75 1.01 0.51 0.10 2.60

Front Front Left 75 0.96 0.50 0.00 2.60

Rear Rear minus Front 86 +0.06 0.34 0.04 0.95 +1.05

Front Rear minus Front 71 -0.21 0.27 0.03 1.15 +0.45 Mean signif. < 0

April 1989 (Strandberg & Junghard, in preparation). Based

on the share of studded tyres in 30 accident involved cars

and in 126 cars bypassing the accident site, the accident risk

was assessed 90% greater for cars without studs. However,

the safety superiority on ice with studded tyres was not

significant (0.1 < p < 0.2), and the relative risk is expected to

be different (from 1.9) when more data are available for

analysis.

References

Aschenbrenner M, Biehl B, Wurm G (1988): Mehr

Vehrkehrssicherheit durch bessere Technik? Feld-

untersuchungen zur Risikokompensation am Beispiel des

Antiblockiersystems (ABS). Unpublished, Mannheim,

1988.

Billing P (1988): Slipperiness. Feature of the Channel 1

program Science . (In Swedish: Halka. Inslag i Kanal

l programmet Vetenskap .) Swedish Television

Broadcasting Company, Stockholm, January 30, 1988.

Glad A (1988): Phase 2 in Driver Education. Effects on

Accident Risk. (In Norwegian: Fase 2 i Föreropplaeringen.

Effekt på ulykkesrisikoen.) Report no. 0015/1988,

Transportokonomisk Institutt, Oslo.

Rompe K, Schindler A, Wallrich M (1987): Advantages

828

of an Anti Wheel Lock System (ABS) for the Average

Driver in Difficult Driving Situations. Proceedings (pp.

442 448) on the 11th ESV Conference, Washington DC,

May 12 15, 1987.

SAE Society of Automotive Engineers (1975): Vehicle

Dynamics Terminology. SAE J670D, Warrendale, PA

15096.

SCB Statistiska Centralbyrån (1986): Traffic Injuries

1985. Official Statistics of Sweden S 1 15 81 Stockholm.

SCB Statistiska Centralbyrån (1987): Traffic Injuries

1986. Official Statistics of Sweden, S l 15 81 Stockholm.

Strandberg L (1983): Danger, Rear Wheel Steering.

Journal of Occupational Accidents, vol. 5, pp. 39 58.

Elsevier Scientific Publishing Company, Amsterdam.

Strandberg L (1988): On Skidding Accidents and

Skidpad Training. Theoretical Analysis and Tests with

Experienced Drivers. (In Swedish: Om sladdolyckor och

halkutbildning. Teoretisk analys och körförsök med vana

forare.) Note TF 60 04, Swedish Road and Traffic Research

Institute, Linköping, October 24, 1988.

Strandberg, L, Tengstrand G, Lanshammar H (1983):

Accident Hazards of Rear Wheel Steered Vehicles. In

Johannsen G & Rijnsdorp J E (eds.): IFAC Analysis, Design

and Evaluation of Man Machine Systems, pp. 399 405,

Pergamon Press, Oxford and New York.