Saturday, May 3, 2008

Review of NCHRP Report 500, Guidance for Implementation of the AASHTO Strategic Highway Safety Plan

I just took a look at NCHRP Report 500, Vol. 18, titled A Guide for Reducing Collisions Involving Bicycles - Guidance for Implementation of the AASHTO Strategic Highway Safety Plan. The lead authors are Craig Raborn of the University of North Carolina Highway Safety Research Center and Darren J. Torbic of the Midwest Research Institute.

In the introduction, the report states

Bicyclists are recognized as legitimate roadway users. ... The safety interests of bicyclists are sometimes in conflict with the interests of motorists. This conflict arises primarily from the substantially different characteristics of the two modes of transportation. Although bicycles can be ridden on most types of roads, the design interests of accommodating higher motor vehicle traffic volumes and speeds during peak hour congestion may create conditions that are less safe for bicyclists. This guide includes road treatments, countermeasures, and other options that support a balanced transportation system.

Safety concerns can significantly influence a person’s decision to bicycle for transportation or recreation. Bicyclists inherently understand that they are vulnerable road users. However, understanding bicyclist safety issues has proven difficult for engineers, planners, and facility designers. Traditionally, safety problems have been identified by analyzing police crash reports, and improvements have been made only after crashes have occurred. Such methods are not sufficient to fully understand and effectively address bicyclist safety concerns; waiting for crashes before responding with countermeasures carries a high price because many bicycle crashes tend to be severe.

First the report says that "bicyclists are legitimate roadway users," then it says that the "design interests of ... higher motor vehicle traffic ... speeds ... may create conditions that are less safe for bicyclists." So the report's stated motivation is safety for bicyclists. (The report says nothing explicit about slower bicyclists delaying higher speed motor vehicles.)

The closest thing I could find of an analysis of why higher traffic speeds create a hazard for bicyclists was this on page III-1:

Speed influences both the severity of crashes that occur as well as the likelihood of occurrence, and has been identified as a contributing factor in all types of crashes. ... Bicyclists are vulnerable road users, and the impact of higher speeds on crash severity is obvious.

Also, in the discussion of Share the Road signs:

The safety effectiveness of shared roadway signs has not been evaluated, and their overall use is thought to be decreasing. Some experts now feel that they are only appropriate in “pinch point” locations where roadway facilities may not fully accommodate both bicyclists and motorists. ... These signs are typically placed along roads with significant bicycle traffic but relatively hazardous conditions for riding, such as narrow travel lanes with no [paved] shoulder, roads or streets with poor sight distance, or a bridge crossing with no accommodation for bicycles.

So hazardous conditions for bicyclists include narrow travel lanes with no paved shoulder. The report does not explain why, other than to say, "the impact of higher speeds on crash severity is obvious."

In analyzing the risks of various types of bicycle accidents, the report states that in a study done in 1996, motorist overtaking cyclist accidents made up 8.4% of total reported bicycle crashes.

The report describes this development of crash type methodology:

The crash typologies developed by Cross and Fisher, by NHTSA, and in the FHWA study evolved into the development of an automated crash typing software, the Pedestrian and Bicycle Crash Analysis Tool (PBCAT) (Harkey et al., 1999), which is currently being further refined for version 2.

Crash Type 11 is

Motorist overtaking bicyclist Description—The motorist was overtaking the bicyclist at the time of the crash.

One of the accompanying figures shows a motorist at night coming upon a lighted cyclist riding on the right edge of a narrow lane in a sharp right hand curve. The implication is that the motorist is about to strike the bicyclist from the rear. In the countermeasures for such an accident scenario, nothing is said of the bicyclist controlling the lane or of the motorist violating the basic speed law by driving too fast for conditions.

The other figure shows a car starting to pass a curb hugging cyclist on a narrow two lane road with a car coming the other way. Again, the report does not propose controlling the lane as a countermeasure. And the report says nothing about motorists thinking that is OK to pass bicyclists anywhere, any time, under any circumstances.

Because the report contains no analysis of the risk of a cyclist controlling a narrow lane being hit from behind, I consider it flawed. It repeats the mistaken analysis that because bicyclists who are hit from behind are at greater risk of serious injury or death, all bicyclists riding in narrow lanes are at greater risk. And because it doesn't analyze overtaking accidents in more detail, it does not address the possible countermeasure of controlling a narrow lane. More seriously, it does not address the countermeasure of integrating bicyclists with traffic.

Instead, the report focuses on countermeasures such as bike lanes and bike boxes that segregate bicyclists from traffic. Until we can gain some influence in the preparation of research reports like this, we will have a limited influence on unsafe practices like the installation of bike boxes in Portland and NYC.

Bob Shanteau

Robert M Shanteau, PhD, PE
Consulting Traffic Engineer
13 Primrose Cir
Seaside, CA 93955-4133
Voice: (831) 394-9420
Cell: (831) 917-0248
FAX: (831) 394-6045