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Investigation of passing accidents using the HSIS data base.

Introduction

The Federal Highway Administration's (FHWA's) study of passing accidents on two-lane, two-way rural roads, discussed on pages 49-60 of this issue, found that the magnitude of passing accidents in one State was very low for the 5-year period 1985 through 1989. The present study was undertaken to determine whether a similar trend in passing accidents could be found in other States.

This study assessed the magnitude of the passing accident problem in two States from 1985 to 1989 on two-lane rural roads and two-lane rural and urban roads. Study results were compared to those from the previous FHWA study. (For clarity, the State cited in that study is here referred to as State A, and the States studied in the present research are here designated as States B and C.) This study was conducted using the FHWA's Highway Safety Information System (HSIS) data base.

Methodology

The Highway Safety Information System has accident and roadway data from five States (Illinois, Maine, Michigan, Minnesota, and Utah) that have been collected over a 5-year period. The data are available in linkable accident, vehicle, roadway, and traffic files. States B and C were chosen for inclusion in this study because of the availability of variables for these States similar to those used to define passing accidents in the State A study.

Table 1 provides a summary of the accident and roadway data available for States A, B and C. As shown in the table, the total number of accidents on all roads and on two-lane rural roads is highest in State A for the 1985 through 1989 period. State B has the second highest number of total accidents, while State C has the second highest number of accidents on two-lane rural roads. Not surprisingly, States A and C also have the highest two-lane roadway mileage. State A's mileage is almost seven times that of State B, and State C's mileage is more than double that of State B. As a result, the average annual accidents on two-lane roads per 100 million vehicle miles (MVM) of travel is higher for State B as compared to States A and C.

The first step in developing a final data set for passing accidents for the three States was to merge the accident, vehicle, and roadway files of all accidents occurring on two-lane roads in the 5-year period. Accidents on unpaved roads were eliminated from the final data set.

The next step was to identify the passing accidents that occurred on the two-lane roads in States B and C. This process entailed defining the specific variables for each State that were to be used in identifying a passing accident.

The vehicle file for State B has two variables that indicate contributing factors to a particular accident (CONTRIB1 and CONTRIB2) and one variable for precrash maneuver (INTENT). An accident was defined as passing if contributing factors indicated "improper overtaking" or precrash maneuver indicated "overtake."

The vehicle file for State C also has two variables indicating contributing factors (CONTRIB1 and CONTRIB2); another variable (MANEUV) is the precrash maneuver of the vehicle. Passing accidents in State C were defined as those whose contributing factors indicated "improper passing" or whose precrash maneuver indicated "overtaking, passing."

Several single-variable tables were next developed from this passing vehicle file to determine whether the remaining accidents were truly passing accidents. These tables revealed that some of the vehicles included in the file had been involved in some kind of turning movement--either a left turn, right turn, or U-turn. Also, several accidents were determined to be either pedestrian or bicycle accidents. The records for turning accidents and pedestrian/bicycle accidents were deleted from the final file.

The final passing accident files for two-lane rural roads contained 495 accidents involving 860 vehicles for State B. State C had 1,422 accidents involving 2,524 vehicles. Figures 1 and 2 show how these numbers relate to the overall numbers of accidents and involved vehicles. The accidents that did not satisfy the constraints of passing accidents were classified as nonpassing accidents.

Comparison Between the States

Indicator Ratios

In order to compare the passing accidents within each State and between the States, two indicator ratios were developed. These ratios, and their values for each of the three States compared in this study, follow:

[Mathematical Expression Omitted]

Comparing the ratios within and between the States reveals the following:

* In all three States, passing accidents are higher on two-lane rural roads than on all two-lane roads.

* In all three States, there are much fewer passing accidents than nonpassing accidents on all two-lane roads.

* State C has the highest percentage of passing accidents followed by State B and then State A.

Comparing the ratios within and between the States reveals the following:

* In all three States, passing accidents are higher on two-lane rural roads than on all two-lane roads.

* In all three States, there are much fewer passing accidents than nonpassing accidents on all two-lane roads.

* State C has the highest percentage of passing accidents followed by State B and then State A.

Severity Distributions

Tables 2 through 7 also show the severity distributions of passing accidents versus nonpassing accidents for States B, C and A on rural and rural/urban two-lane roads.

Comparison of severities within each State shows that passing accidents are somewhat more severe than nonpassing accidents. In State A, the percentage of fatal and incapacitating passing accidents is higher than that of nonpassing accidents. Also, figures 3 and 4 show that fatal and incapacitating passing accidents are highest in State B, followed by State A, and then State C. Non-incapacitating and possible injury accidents are slightly higher in States C and A than in State B. Interestingly, the percentages of non-injury (i.e., property damage only) accidents in each State are quite similar.

Accident Types

Some transportation professionals believe that passing-related accidents are more frequently of a head-on nature than they are of other accident types. To investigate this premise, a set of tables was developed to examine the types of accidents involved in a passing maneuver. The collision type variable was used for State B, the accident type variable was used for State C, and the accident diagram variable was used for State A to develop these tables. Despite the differences in their respective terminology, the variables used by States A and C are actually very similar categories; State B, however, does not have such categories. Instead, a collision type variable was used for State B since this variable indicates the motion of the vehicle at the time of impact. This information is similar to that collected under the State A and C categories.

Table 8 for State B shows that 34.1 percent of the passing accidents are single vehicle, followed by rear-end accidents (25.2 percent) and sideswipe accidents (15.2 percent). Table 9 shows that rear-end accidents in State C predominate (40.4 percent), followed by ran-off-road accidents (30.7 percent) and head-on/sideswipe accidents (18.1 percent). Table 10 shows that sideswipe passing accounts for the largest proportion of passing accidents (31.9 percent)in State A, followed distantly by rear-end accidents (12.2 percent) and head-on accidents (6.4 percent). These results contradict the supposition that passing accidents on two-lane rural roads primarily consist of head-on accidents. All three States show a relatively low percentage of head-on accidents as compared to other accident types.

Conclusion

This study was performed to determine the magnitude of passing accidents on two-lane roads. The data show that the total number of passing accidents with respect to all accidents is in the range of 2 percent to 3 percent for the three States studied. These percentages are very low as compared to other types of accidents; this indicates that passing accidents do not appear to be a significant problem.

The severity of passing accidents is slightly higher than the severity of nonpassing accidents for the three States. Although there are some differences in injury severity among the three States, the total percentages of injury accidents are similar. The States also exhibit similar trends with regard to the severity of their nonpassing accidents. Comparing passing accident type shows that head-on type accidents are less frequent than other types; this finding is contrary to popular belief.

The percentage of passing accidents with respect to all accidents for States B and C reinforces the conclusion of an earlier study made of State A. The low number of passing accidents as compared to total accidents indicates that the potential to improve safety on two-lane roads by changing standards for marking in passing zones is small.

Yusuf M. Mohamedshah is a transportation engineer with Advanced Engineering and Planning Corporation, Inc., (AEPCO) which provides automated data processing support to the Federal Highway Administration (FHWA). Mr. Mohamedshah works with the FHWA's Design Concepts Research Division on its Highway Safety Information System (HSIS) data base. Specifically, he provides technical support to the highway design team in obtaining information from the HSIS data base and in the use of computer-based roadway design packages. Mr. Mohamedshah has a master's degree in civil engineering from Virginia Polytechnic Institution and a bachelor's degree in that field from the University of Bombay, India.
Table 2.---Severity distribution of accidents on
 two-lane rural roads for State B, 1985-89
 Severity Nonpassing Passing All
 accidents accidents accidents
Fatal, incapacitating 12.56% 18.99% 12.71%
 (2,627) (94) (2,721)
Non-incapacitating 10.42% 13.33% 10.49%
 (2,180) (66) (2,246)
 Possible injury 7.36% 11.31% 7.46%
 (1,540) (56) (1,596)
Property damage only 69.65% 56.36% 69.34%
 (14,565) (279) (14,844)
 Total 100% 100% 100%
 (20,912) (495) (21,407)
 Note: Number of accidents are in parentheses.
Table 3.--Severity distribution of accidents on
 two-lane roads (rural & urban)for State B, 1985-89
 Severity Nonpassing Passing All
 accidents accidents accidents
Fatal, incapacitating 9.65% 15.62% 9.73%
 (6,520) (144) (6,664)
Non-incapacitating 10.70% 11.17% 10.70%
 (7,227) (103) (7,330)
Possible injury 12.45% 10.74% 12.45%
 (8,425) (99) (8,524)
Property damage only 67.19% 62.47% 67.12%
 (43,395) (576) (45,971)
 Total 100% 100% 100%
 (67,567) (922) (68,489)
 Note: Number of accidents are in parentheses.
Table 4.--Severity distribution of accidents on
two-lane rural roads for State C, 1985-89
 Severity Nonpassing Passing All
 accidents accidents accidents
Fatal, incapacitating 5.71% 7.45% 5.75%
 (3,008) (106) (3,114)
Non-incapacitating 15.71% 16.17% 15.72%
 (8,277) (230) (8,507)
Possible injury 14.89% 16.46% 14.93%
 (7,846) (234) (8,080)
Property damage only 63.69% 59.92% 63.59%
 (33,560) (852) (34,412)
 Total 100% 100% 100%
 (52,691) (1,422) (54,113)
 Note: Number of accidents are in parentheses.
Table 5.--Severity distribution of accidents on
two-lane roads (rural & urban) for State C, 1985-89
 Severity Nonpassing Passing All
 accidents accidents accidents
Fatal, incapacitating 4.43% 6.28% 4.48%
 (4,366) (146) (4,512)
Non-incapacitating 12.94% 13.38% 12.95%
 (12,739) (311) (13,050)
Possible injury 13.95% 13.94% 13.95%
 (13,735) (324) (14,059)
Property damage only 68.67% 66.41% 68.62%
 (67,611) (1,544) (69,155)
 Total 100% 100% 100%
 (98,451) (2,325) (100,776)
 Note: Number of accidents are in parentheses.
Table 6.--Severity distribution of accidents on
two-lane, rural roads for State A, 1985-89
 Severity Nonpassing Passing All
 accidents accidents accidents
Fatal, incapacitating 9.30% 13.89% 9.37%
 (6,337) (137) (6,474)
Non-incapacitating 16.00% 15.42% 16.00%
 (10,905) (152) (11,057)
Possible injury 15.02% 15.52% 15.03%
 (10,232) (153) (10,385)
Property damage only 59.67% 55.17% 59.60%
 (40,644) (544) (41,188)
 Total 100% 100% 100%
 (68,118) (986) (69,104)
 Note: Number of accidents are in parentheses.
Table 7.--Severity distribution of accidents on
two-lane roads (rural & urban) for State A, 1985-89
 Severity Nonpassing Passing All
 accidents accidents accidents
Fatal, incapacitating 6.65% 12.34% 6.70%
 (13,087) (187) (13.274)
Non-incapacitating 13.97% 14.38% 13.97%
 (27,469) (218) (27.687)
Possible injury 14.46% 13.32% 14.44%
 (28,429) (202) (28.631)
Property damage only 64.92% 59.96% 64.89%
 (127,684) (909) (128,593)
 Total 100% 100% 100%
 (196,669) (1,516) (198,185)
 Note: Number of accidents are in parentheses.


[Tabular Data Omitted]
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Title Annotation:Highway Safety Information System
Author:Mohamedshah, Yusuf M.
Publication:Public Roads
Date:Sep 1, 1992
Words:2076
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