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FACTORS INFLUENCING BICYCLE CRASH SEVERITY

ON TWO-LANE UNDIVIDED ROADWAYS IN NORTH CAROLINA

Jeremy R. Klop

Department of City and Regional Planning

3140 New East Building

University of North Carolina

Chapel Hill, NC 27599

Tel (919) 969-6630, Fax (919) 932-3705,

Email klop@email.unc.edu

http://www.unc.edu/~klop

Asad J. Khattak

Department of City and Regional Planning

3140 New East Building

University of North Carolina

Chapel Hill, NC 27599

Tel (919) 962-4760, Fax (919) 962-5206

Email khattak@email.unc.edu

http://www.unc.edu/~khattak/res951.htm

November 15, 1998

Transportation Research Board

78th Annual Meeting

January 10-14, 1999

Washington, D.C.

Klop & Khattak

Factors Influencing Bicycle Crash Severity

on Two-lane Undivided Roadways in North Carolina

Jeremy R. Klop

Department of City and Regional Planning, University of North Carolina at Chapel Hill

Asad J. Khattak

Department of City and Regional Planning, University of North Carolina at Chapel Hill

ABSTRACT

Concern over crashes involving bicycles and motor vehicles is largely due to the

severity of injuries. This research examines the impacts of physical and

environmental factors on the severity of injury to bicyclists. North Carolina

Highway Safety Information System (HSIS) crash and inventory data for state-

controlled two-lane undivided roadways are analyzed. The injury severity

distribution, measured on the KABCO scale, is as follows: no injury 1.8%;

complaint of pain 24.4%; non-incapacitating injury 42.5%; incapacitating injury

25.5%; and fatal injury 5.9%. The total number of involvements in this dataset

was 1025 with a majority of the involvements occurring outside urbanized areas

(80.5%). Using the ordered probit model, the effect of a set of roadway,

environmental, and crash variables on injury severity is explored. Variables that

significantly increase injury severity include straight grades, curved grades,

darkness, and fog. Higher average annual daily traffic was the only variable

associated with lower injury severity. Separate models are estimated for rural and

urban locations. Marginal effects of each factor on the likelihood of each injury

severity class are reported. Policy implications and possible countermeasures are

then discussed.

KEYWORDS: Injury Severity, bicycle crashes, ordered probit model, North

Carolina

Klop & Khattak

INTRODUCTION

While in the U.S. only 2% of all motor vehicle deaths are bicyclists, 813 bicyclists were

killed in crashes in 1997 alone (1). Urban areas have received the most attention in the

literature, but 36% of all national fatalities occurred in rural areas in 1997 (1). The

interaction between motorists and cyclists on public roadways has been the subject of

increased policy debate in the years following the passage of the ISTEA legislation. An

important component of this discussion relates to cyclist safety and reducing the risk of

injury to users of this mode. While work has been done to describe crash types and the

characteristics of the roadway, vehicles, and persons involved in crashes, analysis of the

factors influencing injury severity is scarce. Because of the complex nature of motor

vehicle/bicycle crashes, understanding the specific factors and their relative influence on

levels of injury severity is useful. This can be accomplished using ordered probability

models, given that injury severity is measured on the KABCO scale in police reports. As

transportation planners and engineers have opportunities to improve facilities to serve

alternate mode choices such as bicycles, this analysis can be especially useful in the

selection and design of facilities which will not only seek to reduce crash incidence, but

also injury severity.

This analysis seeks to examine the influence of roadway and crash variables on

injury severity in motor vehicle/bicycle crashes on state-controlled two-lane roadways in

North Carolina from 1990-1993. Over the four years examined in this dataset, 60

bicyclists were killed in North Carolina, and 947 were injured in police reported motor

vehicle/bicycle crashes. Additionally, North Carolina maintains a system of State Bicycle

Routes of over 2,250 miles, 90-95% of which is on two-lane undivided roadways in rural

areas (2). Relatively good quality data related to the crashes on these roadways was

available from The Federal Highway Administration’s HSIS (Highway Safety

Information System) database, which is used for analysis. The North Carolina Division

of Highways maintains a particularly extensive roadway inventory system for 77,000

miles of roadway, representing a very high proportion of the total mileage existing in the

92,000 state miles. Approximately 34,800 have been entered into a computer

mileposting system, so crashes can be linked to them (3). Also there are no county

maintained roadways in North Carolina, so most rural roadways are contained in the

dataset used in this study.

LITERATURE REVIEW

The definitive early work in the field of bicycle safety by Cross and Fisher (4) served to

identify crash types and median ages of the cyclists involved and the severity of injury

(fatal or non-fatal) in each crash type. The results were especially useful in targeting

educational programs toward age groups that were over-represented in certain crash

types. Additional Federal research in the early 1980s focused on cyclist behavior,

analysis of specific crash types such as right-turn-on-red situations, and development of

countermeasures (5,6,7,8). Stutts and Hunter (9) review these efforts as well as a

considerable number of safety education efforts undertaken both by Non-Governmental

Organizations and a number of States in the 1980s.

Klop & Khattak

Considerable work has been done by the FHWA to compile descriptive statistics

on both pedestrian and bicycle crash types. In a FHWA-sponsored report on Pedestrian

and Bicycle Crash Types of the Early 1990’s (10), Hunter et al., analyzed hard copies for

3000 bicycle/motor vehicle crashes from six states and linked 2,990 of them with

computerized state police report files. The major findings are as follows. Intersections,

driveways and other junctions account for three fourths of all crashes. The under 15 ages

are over-represented and older adults 25-44 and 65+ are underrepresented in crashes, but

44+ are over-represented in fatalities. 18% of all bicycle involvements are serious or fatal

(A+K). Alcohol or drug use is present in about 5% of crashes overall, but is 15% for the

25-44 group, and is more frequent on weekends and hours of darkness. Two-thirds of the

involvements occur during late afternoon and early evening (exposure is quite high and

visibility can be a problem during these times). Two-thirds of the involvements are

urban, and 7% are on private property. Ages 10 and under are over-represented in

driveways, alleys and parking lots. About 60% of the involvements occur on two-lane

roadways while roads with narrower lanes and higher speed limits are associated with

more than their share of serious and fatal injuries to bicyclists.

Research related to facility location, selection, and design for bicyclists is still

developing. The 1984 AASHTO “Green Book” (11) recommends paved shoulders, wide

outside traffic lanes (15ft minimum), bicycle-safe drain grates, and maintenance of

smooth, clean riding surfaces. More specific guidelines are available in the 1991

AASHTO “Guide for the Development of New Bicycle Facilities” (12). Regarding lane

widths, McHenry and Wallace (13) found that in multi-lane highways, optimal lane width

was greater than 13 ft 8 in., and less than 17 ft. A 1994 FHWA-sponsored study tour of

England, Germany, and the Netherlands examined pedestrian and bicyclist safety

conditions (14). They found that reduced vehicle speeds were of major importance to

bicyclist and pedestrian safety, in addition to restricted traffic movement and reduced

travel distances. Specific facility related bicycle safety measures are also discussed,

though primarily in an urban context.

Relevant exposure data are often not available, and there is a clear need for

collecting such data in bicycle safety research. In a recent paper examining exposure and

safety on roads, off-road paths, and sidewalks, Aultman-Hall and Hall found that the rate

of major injuries was highest on sidewalks and off-road paths, compared to on-road

cycling (15). The analysis also demonstrates one method for quantifying bicycle travel

exposure rates. Assessing the level of risk associated with particular locations, behaviors,

and conditions is difficult without information about how many cyclists are uninjured as

well as injured. This analysis will focus on factors influencing severity once a crash

occurs.

DATA DESCRIPTION & HYPOTHESES

In this study, it is assumed that for crashes involving a motor vehicle and a bicycle, the

cyclist is more severely injured (98% of drivers sustained no injury in the 2,990 cases

examined by the FHWA Pedestrian and Bicycle Crash Types of the Early 1990’s). The

significant difference in mass between the motor vehicle and the bicycle, as well as the

protection afforded the motorist by the physical structure of the vehicle also contribute to

Klop & Khattak

increased injury severity for the cyclist compared to the motorist. Therefore, the severity

variable, representing the worst injury in the crash, is assumed to refer to the cyclist.

Roadway and environmental factors as well as individual, vehicle, and bicycle

factors drive the collision process. During a crash event, the energy transfer determines

injury severity in the collision and possible fall from the bicycle, along with the medical

care that is subsequently administered. Reaction times of both the motorist and the

cyclist, perceptual and judgement errors, and attention also affect the collision process.

Information processing of both the cyclist and the driver, the care that some drivers use

when near bicyclists, and the risk taking behaviors by both the motorist and bicyclist may

also increase or decrease injury severity.

While the literature classifies over 80 crash types (10), the HSIS data set used for

this analysis does not disaggregate by crash type. Additionally, in this dataset, no

information is collected on bicycle characteristics, and individual characteristics are

limited to age, gender, and race on the crash reporting form. However, detailed

information about roadway and environmental characteristics is gathered for all crashes.

The severity of injury is expected to increase with the motorist’s lack of awareness of the

cyclist and inability to maneuver because of roadway or environmental characteristics.

Additionally, while driver and cyclist characteristics, as well as vehicle characteristics

may contribute to increased or decreased severity, this analysis is limited to physical and

environmental characteristics. Since the roadway characteristics are policy sensitive, they