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Risk Factors for Bicycle-Motor Vehicle Collisions at Intersections*

 

BY ALAN WACHTEL AND DIANA LEWISTON

In 1992, 722 bicyclists were killed in the United States in collisions with motor vehicles1, and an estimated 650,000 people were treated in emergency rooms for bicycle-related injuries.2 It is remarkable that, for a traffic safety problem of this magnitude, so little research has been conducted to establish the causes of these acci­dents. Instead, design standards for roadways and bicycle facilities, individual project designs, and laws and policies regarding bicycling are based almost entirely on opinion. The quality of the results is highly variable.

This paper reports a study of bicycle-motor vehicle collisions in the city of Palo Alto, Cali­fornia. The study compares personal character­istics and bicycling behavior-age, sex, direction of travel (with or against traffic flow), and posi­tion on the road (roadway or sidewalk) of bicy­clists involved in accidents with similar data for the general population of bicyclists observed along the same streets. This comparison enables us to identify factors that are correlated with increased risk of bicycle-motor vehicle colli­sions, and to suggest engineering practices that reduce this risk.

Methods

Accident Records

From 1981 to 1990, one of the authors, Diana Lewiston, analyzed all police reports of bicycle accidents in Palo Alto. This study considers only the period from July 1985 through June 1989. (Earlier data were entered in an incompatible computer format and are no longer available.) During this period, bicycle-motor vehicle colli­sions accounted for 314 of 371 bicycle accidents for which a substantially complete police report was available (85 percent). The remaining acci­dents involved single bicycles, or collisions with another bicycle, a pedestrian, or, in one case, a train, which resulted in the only fatality during the study period. Since they constitute the majority of all reported bicycle accidents, this study considers only the incidence of bicycle-motor vehicle collisions.

Bicycle accidents at intersections accounted for 237 of 371 total bicycle accidents (64 per­cent), and 233 of 314 bicycle-motor vehicle col­lisions (74 percent). We define an intersec­tion broadly as any point where turning or crossing movements are possible for the bicy­clist or the motorist. The definition therefore includes not only the junction of two roadways, but also points where driveways, sidewalks, or paths meet a roadway, or where sidewalks or paths meet a driveway.

The large fraction of accidents that occurred at intersections indicates that these are the major points of conflict between bicyclists and motor­ists. Overtaking accidents, in which a bicyclist in the roadway was struck from behind by a motorist traveling in the same direction, accounted for only 5 of 314 bicycle-motor vehicle collisions, and sideswipes for 8. The remaining non-intersection collisions included those in which a bicyclist overtook a parked or parking motor vehicle, a motorist opened the door of a parked car into the bicyclist's path, or a motorist or bicyclist changed lanes improperly.

Table 1. Percentage Distribution of Intersection Accidents by Bicyclist Characteristics

There is no national reporting system for bicycle-motor vehicle accidents. If an accident is fatal, however, it is almost always well docu­mented and reported. The Fatal Accident Reporting System (FARS) of the National Highway Traffic Safety Administration (NHTSA) reported that only 31 percent of bicy­clist fatalities in motor vehicle accidents in the United States in 1992 occurred at intersections.1 NHTSA's classification follows the Manual on Classification of Motor Vehicle Traffic Acci­dents (ANSI D16.1-1989)3, which defines an intersection as a crossing of two or more road­ways not classified as driveways. Our use of intersection corresponds more nearly to the Manual's “junction,” defined as either an inter­section or the connection between a driveway and a roadway. FARS statistics for 1992 show 39 percent of fatalities at junctions.4

In urban areas the value increases to 44 percent, somewhat closer to our findings. It is possible that non-intersection accidents are more likely to result in fatalities.

Table 1 shows the distribution of bicycle-motor vehicle collisions at intersections, cata­logued according to four characteristics that are easily observed and might be relevant for acci­dent risk: bicyclist age, bicyclist sex, direction of bicyclist travel (with or against the direction of traffic on the roadway), and bicyclist position (either in the roadway, including bicycle lanes and private driveways, or on the sidewalk, including bicycle paths and crosswalks).

The table shows that 35 percent of victims were aged 17 or younger, while 65 percent were 18 or older, and that 31 percent were female and 69 percent were male. It is obviously not possi­ble to conclude from these figures that older bicyclists or male bicyclists are at greater risk: the actual risks depend on the age and sex distri­bution of the bicyclist population that is exposed to potential accidents. For the same reason, it is impossible to draw any conclusions about the risks involved in bicycling with or against the direction of traffic, or on the roadway or the sidewalk, without knowing how many bicyclists in each category were exposed.

Exposure Counts

In order to study the distribution of these four characteristics in the population of bicyclists that is exposed to accidents, the City of Palo Alto’s Transportation Division arranged to conduct bicyclist counts in May 1987, including counts at intersections along three major arterial streets, Middlefield Road, Embarcadero Road, and El Camino Real, on which many bicycle accidents had occurred (92 of 233 bicycle-motor vehicle intersection accidents). Table 1 shows that the distribution of the selected bicyclist characteris­tics in accidents along these streets is similar to that in the entire city.

Middlefield Road is a residential street, except for one neighborhood shopping center and a two-block business district. It varies from two to four lanes in width, carries about 16,000 motor vehicles per day, and has on-street bicycle lanes for a portion of its length. The posted speed limit is 25 mi/h. Embarcadero Road is a four-lane residential street carrying about 22,000 motor vehicles per day; the posted speed limit is 25 mi/h, but the measured 85th percentile speed is 37 mi/h. It includes a small neighborhood shopping center at one end and a moderate-sized shopping center at the other and, opposite it, a high school.

Table 2. 18 and Older Compared to 17 and Younger

	18 and Older			17 and Younger			Risk Ratio
Category	Bicyclists Observed	Accidents Reported	Risk	Bicyclists Observed	Accidents Reported	Risk	18 to  17	p
All bicyclists	1543	59	1.3	1433	30	0.7	1.8	0.01
Female	363	15	1.4	489	7	0.5	2.9	0.03
Male	1180	44	1.2	944	23	0.8	1.5
With traffic	1418	45	1.1	1135	11	0.3	3.3	0.001
Against traffic	125	14	3.7	298	19	2.1	1.8
Roadway	1265	39	1.0	740	9	0.4	2.5	0.02
Sidewalk	278	20	2.4	693	21	1.0	2.4	0.01

 

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Table 3. Male Compared to Female

	Male			Female			Risk Ratio,
Category	Bicyclists Observed	Accidents Reported	Risk	Bicyclists Observed	Accidents Reported	Risk	Male to Female	p
All bicyclists	2124	67	1.1	852	22	0.9	1.2
17	944	23	0.8	489	7	0.5	1.7
18	1180	44	1.2	363	15	1.4	0.9
With traffic	1819	43	0.8	734	13	0.6	1.3
Against traffic	305	24	2.6	118	9	2.6	1.0
Roadway	1448	35	0.8	557	13	0.8	1.0
Sidewalk	676	32	1.6	295	9	1.0	1.6

Table 4. Against Traffic Compared to With Traffic

	Against Traffic			With Traffic			Risk Ratio,
Category	Bicyclists Observed	Accidents Reported	Risk	Bicyclists Observed	Accidents Reported	Risk	Against to With	p
All bicyclists	423	33	2.6	2553	56	0.7	3.6	<<0.00001
Roadway	108	5	1.5	1897	43	0.8	2.0
Sidewalk	315	28	3.0	656	13	0.7	4.5	<0.00001
17	298	19	2.1	1135	11	0.3	6.6	<<0.00001
18	125	14	3.7	1418	45	1.1	3.5	0.0001
Female	118	9	2.6	734	13	0.6	4.3	0.001
Male	305	24	2.6	1819	43	0.8	3.3	<0.00001

Table 5. Sidewalk Compared to Roadway

	Sidewalk			Roadway			Risk Ratio,
Category	Bicyclists Observed	Accidents Reported	Risk	Bicyclists Observed	Accidents Reported	Risk	Sidewalk to Roadway	p
All bicyclists	971	41	1.4	2005	48	0.8	1.8	0.01
17	693	21	1.0	740	9	0.4	2.5	0.03
18	278	20	2.4	1265	39	1.0	2.3	0.01
Female	295	9	1.0	557	13	0.8	1.3
Male	676	32	1.6	1448	35	0.8	2.0	0.01
With traffic	656	13	0.7	1897	43	0.8	0.9
Against traffic	315	28	3.0	108	5	1.5	1.9
17, female	225	4	0.6	264	3	0.4	1.6
17, male	468	17	1.2	476	6	0.4	2.9	0.04
18, female	70	5	2.4	293	10	1.1	2.1
18, male	208	15	2.4	972	29	1.0	2.4	0.01
17, with traffic	455	5	0.4	680	6	0.3	1.2
18, with traffic	201	8	1.3	1217	37	1.0	1.3
17, against traffic	238	16	2.2	60	3	1.7	1.3
18, against traffic	77	12	5.2	48	2	1.4	3.7
Female, with traffic	210	2	0.3	524	11	0.7	0.5
Female, against traffic	85	7	2.8	33	2	2.0	1.4
Male, with traffic	446	11	0.8	1373	32	0.8	1.1
Male, against traffic	230	21	3.1	75	3	1.3	2.3
17, female, with	159	0	0.0	244	2	0.3	0.0
17, female, against	66	4	2.0	20	1	1.7	1.2
18, female, with	51	2	1.3	280	9	1.1	1.2
18, female, against	19	3	5.3	13	1	2.6	2.1
17, male, with	296	5	0.6	436	4	0.3	1.8
17, male, against	172	12	2.3	40	2	1.7	1.4
18, male, with	150	6	1.3	937	28	1.0	1.3
18, male, against	58	9	5.2	35	1	1.0	5.4

 

Portions of Middlefield and most of Embar­cadero are too narrow to accommodate bicycle lanes; accordingly, the city has designated side­walks in these places as bicycle paths. (Bicycle lanes are portions of the roadway designated for the use of bicycles. Bicycle paths are physically separated rights of way for the exclusive use of bicycles and pedestrians.) The paths are signed “Bicycles May Use Sidewalk,” and their use is optional. In accordance with a local ordinance these sidewalks are further signed for one-way bicycle travel, although this prohibition is often ignored and rarely enforced.

El Camino Real is a six-lane divided state highway (Route 82) located primarily in a busi­ness district, with parking permitted and many commercial driveways. It carries about 46,000 vehicles per day at a posted speed limit of 35 to 40 mi/h and has no bicycle facilities.

Middlefield and Embarcadero have continu­ous sidewalks on both sides, and El Camino Real has them for most of its length in the city.

Bicyclists were counted at four intersections along Middlefield Road, at two intersections along Embarcadero Road, and at three intersec­tions along El Camino Real. The intersections chosen offered a representative mixture of arterials, collectors, and neighborhood streets; adult commuters, college students, and school­children; and on-road bicycle lanes, sidewalk bicycle paths, and roadways without special bicycle facilities. All but two intersections were signalized; these two had stop signs on the minor street.

Nearly 3000 cyclists were observed during a one-day count of 8 hours at each intersection. For each cyclist entering any leg of the intersec­tion, observers trained by the Transportation Division collected data on approximate age (estimated as either 17 years of age and under or 18 and older), sex (male or female), direction of travel (with or against the direction of traffic on the roadway), and position (either in the road­way, including bicycle lanes, or on the sidewalk, including bicycle paths and crosswalks).

Data Analysis

Data analysis is based on figures for the May 1987 bicyclist counts and for July 1985–June 1989 police-reported accidents, extending approximately two years before and two years after the exposure counts. To eliminate as many extraneous influences as possible, the accidents analyzed were restricted to those that took place at intersections along the three arterial streets where the counts were made. Of 92 such acci­dents, information for all four variables was available for 89; only these 89 accidents are analyzed here. The results identify risk factors for bicycle-motor vehicle collisions at intersec­tions.

We quantify the risk of a bicycle-motor ve­hicle collision in two ways. First, we define the risk for any group of bicyclists as (a/A)/(b/B), where a is the number of accidents that occur to the group, A is the total number of accidents, b is the number of bicyclists in the group, and B is the total number of bicyclists. In this study A = 89 and B = 2976. Risk is proportional to the accident rate per bicyclist: the lower the risk, the lower the likelihood of an accident. By defini­tion, the average risk of all bicyclists in the study is exactly 1, in arbitrary units.

We also make a number of binary compari­sons between groups, by calculating the ratio of their risks. We test this ratio for statistical significance by calculating the expected number of accidents for each of the two groups, based on the assumption that accidents should be distrib­uted in the same proportion as exposures. We then compare the number of accidents expected to the number observed, using a 2 test with Yates’s correction for continuity and one degree of freedom. This test determines the probability p that any discrepancy (equivalent to a risk ratio different from 1) is due to chance rather than to a real difference in risk. We report the result as an upper bound, and only when p<0.05. If p<0.01 the upper bound is given only as the next higher power of ten.

The analysis sums accident and exposure data from Middlefield, Embarcadero, and El Camino Real. Because the risk of a bicycle-motor vehicle collision should be proportional to the number of motor vehicles as well as to the number of bicyclists, these three streets, which have different traffic volumes, might be expected to have different accident rates per bicyclist, and it might therefore be misleading to combine data from them. Analysis of the three corridors separately, however, shows that the overall risk (as defined above) along Middlefield is 1.08, along Embarcadero 0.97, and along El Camino 0.96—for all practical purposes identi­cal. For the four major binary comparisons listed in the next section, “Results,” we have also analyzed the data for each corridor independ­ently; we find that, although the risks and risk ratios naturally vary somewhat from corridor to corridor, the same patterns emerge. We therefore feel confident that no errors are introduced by combining the three corridors in order to