An investigation of passing accidents on two-lane, two-way roads

Public Roads, Sept, 1992 by Henrietta B. Alexander, Paul A. Pisano

Introduction

The passing maneuver on two-lane highways is one of the most demanding and hazardous operations performed by motorists. Its danger lies in the fact that a passing vehicle must occupy an opposing lane of traffic to complete the maneuver, and it is not uncommon for drivers at some point in time to want to pass a slower moving vehicle on a two-lane highway. Two-lane highways comprise the majority of the roadway mileage in the United States. On the Federal-aid Highway System alone, nearly 960,000 km (600,000 mi) are classified as two-lane rural highways, and an additional 193,000 km (120,000 mi) are classified as two-lane urban highways. (1) [1]

A recent study undertaken by the Federal Highway Administration (FHWA) analyzed the issues surrounding passing on two-lane rural roads. The goals of this research were to:

* Determine if there is a safety problem in passing zones on two-lane, two-way highways.

* Determine the magnitude of the problem and associated risk.

* Identify causal and/or contributory factors to the problem.

Background

Since the 1920's, much has been written about passing problems on two-lane rural roads. (2, 3, 4) A critical issue that was identified in the past research is the difference between the American Association of State Highway and Transportation Officials (AASHTO) passing sight distance design criteria for two-lane roads found in the manual A Policy on Geometric Design of Highway and Secondary Streets and the marking criteria for no-passing zones on the same roads found in the Manual on Uniform Traffic Control Devices for Streets and Highways (MUTCD). (5, 6) These criteria are presented in tables 1 and 2, respectively.

In the field, passing zones are not specifically marked; instead, they exist where no-passing zones are not warranted. The warrants for no-passing zones are established by the MUTCD, and the MUTCD values are clearly different from the AASHTO values, which are used in the design of two-lane roads and not for traffic control. The net result was that two different sets of "minimum passing sight distances" were developed. However, the FHWA and others recognized that the values in the MUTCD are not minimum passing sight distances, but rather they are minimum sight distance requirements. Furthermore, it was recognized that these two sets of values serve two different functions. (4)

Many past studies investigated the validity and practicability of these two sets of requirements. The general consensus was that the MUTCD values were too low. The AASHTO requirements were also questioned by some researchers as too conservative; others concluded that they were inadequate for car-passing-truck and truck-passing-truck scenarios.

Other issues were overlooked or not addressed by past studies on this topic. For example, there was no complete investigation of accidents relating to passing zone pavement markings to determine if there is a passing zone safety problem, nor was the magnitude of the problem documented. Also, only a few investigations were conducted to determine and analyze the casual and/or contributory factors of passing accidents.

Assessment of previous studies identified the need to do a detailed analysis of passing accidents with respect to the above-described factors to determine if there are deficiencies in current practices for marking passing zones as specified in the MUTCD. In other words, the intent of this study was not to compare the AASHTO values to the MUTCD values but rather to assess the level of safety of the two-lane roads with respect to the vehicles passing each other as defined by the MUTCD. If deficiencies were identified, then steps would be taken to correct the problem. If it was determined that current marking practices do not create unsafe driving conditions, then other concerns, including those related to the different sets of sight distances, could be resolved.

Research Scope

The scope of this research project consisted of the study of the accident experience of two-way, two-lane highways with passing zones marked in accordance with MUTCD values. To fulfill study objectives, data were needed on passing zones that included a large number of accident, traffic, roadway, environmental, and vehicle factors. Information on passing zone pavement markings and their relationship to the accident data was also required. The only viable sources for these types of data are State files. Since the FHWA Highway Safety Information Systems (HSIS) has been developed using State data, it was selected for use in this project.

The HSIS is a roadway-based system that provides quality data on a large number of accidents, vehicles, drivers, roadway segments, and other roadside inventories for five States. Six years of data from 1985 through 1990 are currently available for each State. At the time that this research was conducted, data were available for the years 1985 through 1989. The five States were comparatively reviewed to determine the amount, nature, availability, and workability of variables and data points in order to select the State(s) to be used in this study.