Lec 33, Ch.5, pp.147-164: Accident reduction capabilities and effectiveness of safety design features (Objectives)

Learn what’s involved in safety engineering studies

Learn how to compute accident reduction capabilities of countermeasures

Learn how to estimate the effectiveness of safety design features (Reduction of the number of accidents)

What we discuss in class today…

Components of engineering studies Condition diagram and collision

diagrams Accident reduction capabilities of

countermeasures Accident reduction factors – definitions Accident reduction factors relating to

improvements to roadway cross section

Component of the Highway Safety Improvement Program (HSIP) by FHWA

Need estimates of the effectiveness of safety design features

Conducting engineering studies (after hazardous locations have been identified)

In-depth study of the accident data obtained for the study site

Conduct a field review of the study site List possible accident (contributory) causes Determine specific safety deficiencies at the

site Develop general countermeasures Conduct an economic analysis (cost-

effectiveness, rather than cost-benefit) Recommend a list of countermeasure

actions

Steps:

Site analysis – Draw a condition diagram

The first thing you do is visit the site and prepare a condition diagram of the site.

Purposes:

To identify contributing causes

To develop site specific improvements

Two types of info:

Accident data

Environment & physical condition data

Site analysis (cont) – Prepare a collision diagrams

Site analysis (cont) – Questions to askGroup accidents by type and answer the following 3 questions, which will lead you to possible countermeasures. See Table 5.3.

What driver actions led to the occurrence of such an accident?

What conditions existing at the location could contribute to drivers’ taking such actions

What changes can be made to reduce the chance of such actions occurring in the future?

Rear-end collisions:

Driver: Sudden stop & Tailgating

Environment: Too many accesses and interactions with vehicles in/out of the accesses (drive ways), bad sight distance, short/long yellow interval, inappropriate location of stop lines (against driver expectancy), etc.

Crash reduction capabilities Used to estimate the expected reduction in crashes that will occur during a given period as a result of implementing a proposed countermeasure. CR = crash reduction (CR) factors are used to indicate potential crash reduction capabilities.

periodbeforeADT

periodafterADTCRNpreventedCrashes__

___

N = expected number of crashes if countermeasure is not implemented and if the traffic volume remains the same.Example 5-

5: CR = 0.3, ADT before = 7850, ADT after = 9000, No. of specific types of crash occurring per year = 12, 14, 13 for the same 3 years where ADT average values were computed.Avg no. of crashes/year = (12+14+13)/3=13Crashes prevented = 13 x 0.3 x (9000/7850) = 4.47 say, 4 accidents

Procedure to determine Crash reduction factor (CR)

mm CRCRCRCRCRCRCRCRCRCR

)1)...(1(...)1)(1()1(

11

321211

When multiple countermeasures are selected…

CR = overall crash reduction factor for multiple mutually exclusive improvements at a single siteCRi = crash reduction factor for a specific countermeasure im = number of countermeasures at the siteExample 5-

6CR1 = 0.40, CR2 = 0.28, and CR3 = 0.2. Determine the overall CR factor. Note that countermeasures are ordered in the descending order of their accident reduction factor values.CR = 0.4 + (1 – 0.4)*0.28 + (1 – 0.4)(1 – 0.28)*0.2

= 0.66

Effectiveness of safety design features (eventually we want to estimate the number of crashes that can be prevented (CP).)

In this chapter, we will see how (1) access control, (2) alignment, (3) cross sections, (4) intersections, and (5) pedestrian and bicyclist facilities might affect the overall safety of roadways. Among these cross section related factors are used as an example to compute CP values.

Access Control: Defined as “some combination of at-grade intersections, business and private driveways, and median crossovers”

e.g. interstates

Streets

More access control Less accidents

Access control (cont)Some methods to reduce crashes by controlling access: Remove access points (remove median openings) Provide frontage roads for business access Provide special turning lanes (TWLTL or LT bays) Warn motorists of changing conditions along the roadway using proper traffic control devices(Note that the access

control section of the chapter does not give CR values.)

More access, higher crash rates

Alignment (This topic was discussed in Ch. 16. Review that chapter to find out what affected vertical and horizontal alignment design.)

Vertical alignment Most important factors include sight distance (especially crest vertical curves) and the vertical curve length.

Improvements to safety of horizontal curves include: Use a less sharp H-curve Widen lanes and

shoulders Add spiral transition

curves Increase the amount of

superelevation < max allowed

Increase the clear roadside recovery distance

Improve the combination of V- and H-curves

Assure adequate pavement surface drainage

Provide increased skid resistance(Note that the alignment

section of the chapter does not give CR values.)

Cross sections (this section gives CR values)

Clearance

(CR values)

Cross sections (cont)

(AR values)

(CR values) for shoulders

(Combined effects)

Cross sections (cont)

(CR values)

Table 5.11 is slightly different from other tables. It does not give CR values. It gives % or cross-section related crashes (RC values) including run-off-road, head-on, and opposite- and same-direction sideswipe.

Table 5.11 Ratio of Cross Section Related Crashes to Total Crashes on Two-Lane Rural Roads

Auxiliary lanes can reduce crashes (because they provide safer passing opportunities.

F = fatal accidentsI = injury accidents

(CR values)

Example 5-7Given: A two-lane two-way highway in mountainous terrain 53 crashes per year (3 year average) Currently 10-ft wide lane, 2-ft unpaved shoulder ADT = 4000 vpd

Improvement options: Widen 10-ft lane to 12-ft lane (2 ft increase) Widen unpaved 2-ft shoulder to paved 6-ft shoulder A combination of the two options

Find the expected number of accidents reduced:RC = 53 x 0.61 = 32 related crashes (Tab 5.11)

a. Crashes prevented (CP) by lane widening = 32*0.23 = 7 accidents/yr (Tab 5.8)

b. CP by shoulder widening = 32*0.29 = 9 accidents/yr (Tab 5.9)

c. CP by the combination = 32*0.46 = 15 accidents/yr (Tab 5.10)