Leonard Evans

RISKY DRIVING RELATED TO DRIVER AND VEHICLE CHARACTERISTICS

This investigation was performed to determine relationships between driver and vehicle characteristics and risk taking in everyday driving, as measured by close following in freeway traffic. A total of 12,000 observations were made of headways (time intervals between successive vehicles) in high flow freely moving freeway traffic at two sites, one in Michigan, the other in Toronto, Canada. The headways were measured using a photographic technique which allowed vehicle and occupant characteristics to be recorded, including type of vehicle and sex and seat belt use. For the Michigan observations, the vehicle license number was recorded, from which additional information on the vehicle and owner (usually the driver) was obtained from state files, including the vehicle mass and model year and the owners age. sex and history of recorded accidents and traffic violations. Shorter headways (close following), corresponding to higher risk, were found for drivers with prior accidents or violations, young drivers, male drivers, drivers with no passengers. and drivers who did not wear a seat belt. Vehicles associated with shorter headways included newer cars and cars of intermediate mass (1600-1900 kg).

HUMAN BEHAVIOR AND TRAFFIC SAFETY

Contains the paper and discussions from the symposium on human behavior and traffic safety, held at the general motors research laboratories on September 23-25, 1984.

Involvement of older drivers in multivehicle side-impact crashes

Side impacts were studied using three separate analyses. National Accident Sampling System (NASS) and National Crash Severity Study (NCSS) cases were reviewed on multivehicle crashes involving fatal chest and abdominal injury by interior contact. Twenty-five cases were analyzed and showed an unusually high involvement of older occupants. Analyses of the 1975-1986 FARS confirmed an overinvolvement. Sixty-four percent of near-side seated occupants were over 50 years old and 36% over 70 in fatal multivehicle side impacts. In contrast, 26% of victims in single-vehicle frontal crashes were over 50 and 8% over 70 years old. Analysis of the 1982-1986 NASS showed that single-vehicle side impacts are not an important injury risk for older drivers, except on icy or wet roads. In contrast, the risk of injury in multivehicle side impacts increases steadily with age and is a major problem for older drivers. The individual NASS and NCSS cases also showed that 88% of the multivehicle side crashes took place at an intersection and that the driver of the struck vehicle frequently caused the crash by driving error (48%) or traffic violation (16%). The majority of cases occurred in daylight hours, on dry roads, and without alcohol involvement. Changes in visual perception, judgment and attention of the older driver may be factors in their missing a traffic signal or turning in front of traffic under the right-of-way. In addition, a reduced tolerance to impact force probably contributes to the injury. Although an analysis of photographs of the side-impacted vehicle indicated that 44% had side-structure deformation that was similar to that produced in the National Highway Traffic Safety Administration (NHTSA) moving deformable barrier test, only 24%-32% of the cases actually addressed the proposed NHTSA dynamic side-impact test. The results of this analysis bear on the agencys preliminary regulatory impact analysis.

SAFETY-BELT EFFECTIVENESS: THE INFLUENCE OF CRASH SEVERITY AND SELECTIVE RECRUITMENT

While theoretical considerations show that the effectiveness of occupant protection devices declines from 100% at very low crash severity to 0% at high severity, empirical details have been lacking. When overall in-use effectiveness is estimated by applying traditional methods to data sets that lack a measure of severity, large biases are introduced because non-wearing drivers are riskier drivers, an effect that has been called selective recruitment. These effects are investigated empirically using National Accident Sampling System (NASS) data in which crash severity is measured by delta-v, the estimated change in the speed of the car as a result of the crash. Supplemental results are obtained using published police-reported data containing a more easily obtained but less objective severity measure. Both data sets provide information on driver fatalities and injuries, thus allowing four comparisons of effectiveness estimates based only on total casualties with ones taking into account the different severities of crashes by belted and unbelted drivers. The data show consistently that the probability that a driver is belted declines as crash severity increases. Belt effectiveness estimates ignoring this effect are biased upwards by large amounts (for example, 60% compared to 40% for injuries using NASS data). Belts appear more effective at preventing fatalities than at preventing injuries. The results are consistent with a prior estimate, derived using a method unaffected by the biases discussed here, which found that, averaged over all crashes, safety belts reduce driver fatality risk by (42 +/- 4).

Seating position in cars and fatality risk

Fatality risk in passenger cars according to seating position (front versus rear; left versus center or right) was examined using Fatal Accident Reporting System (FARS) data for 1975 through 1985. Comparing the fatality risk of unrestrained occupants matched in sex and age (within three years) revealed effects attributable to seating position, and not to occupant characteristics correlated with use of different seats. Fatality risk to drivers was the same as fatality risk to right front passengers to within 1 per cent; this was so for crashes in all directions and for frontal crashes. Fatality risk in rear seats was (26 +/- 2) per cent lower than in front seats, and lower in center compared to outboard seats by (22 +/- 4) per cent for front seats and (15 +/- 4) per cent for rear seats. The center rear seat was associated with the lowest fatality risk.

Do accident-involved drivers exhibit riskier everyday driving behavior?

Abstract For more than forty years researchers have tried to relate traffic accident involvement to how an individual normally drives. Such efforts have not hitherto discovered clear effects. This study establishes statistically significant relations between prior involvement in accidents and an observed characteristic of every day driving. The observed driving characteristic is following headway in high flow freeway traffic, defined as the time interval between a vehicle and the preceding vehicle in the same lane. This headway is interpreted as a measure of driver risk. It is found that accident-involved drivers are more likely to follow with short headways (less than 1 s) than accident-free drivers. A similar effect is found in comparing drivers with and without traffic violations. Records of accident involvement and traffic violations were obtained (with the cooperation of the Michigan Department of State) for 2576 unidentified drivers observed travelling on a local freeway in high flow afternoon rush hour traffic. The observational data consisted of a measure of the headway and a photograph of each vehicle from which the license plate number was read. In most cases the license plate number provided the driver license number of the registered owner. Cases were excluded if the sex and age of the driver observed in the photograph did not match the owner information. The traffic violation and accident records available from the driver license file were used to examine following headway as a function of driver record. Based on their observed closer following in freeway traffic it is concluded that accident-involved drivers and drivers cited for violations exhibit higher levels of risk in everyday driving than accident-free and citation-free drivers.

Restraint effectiveness, occupant ejection from cars, and fatality reductions.

The effectiveness of air cushion restraint systems, or airbags, in preventing fatalities is estimated by assuming that they do not affect ejection probability, and protect only in frontal, or near frontal, crashes with impact-reducing effectiveness equal to that of lap/shoulder belts. In order to compute airbag effectiveness, lap/shoulder belt effectiveness and the fraction of fatalities preventable by eliminating ejection are estimated using Fatal Accident Reporting System (FARS) data. Ejection prevention is found to account for almost half of the effectiveness of lap/shoulder belts (essentially all for lap belts only). Airbag effectiveness is estimated as (18 +/- 4)% in preventing fatalities to drivers and (13 +/- 4)% to right front passengers. Drivers switching from lap/shoulder belt to airbag-only protection increase their fatality risk by 41%.

Driver injury and fatality risk in two-car crashes versus mass ratio inferred using Newtonian mechanics

This paper aims at explaining the results of a recent empirical study that found that when cars of unequal mass crash into each other, the ratio of driver fatality risk in the lighter care to risk in the heavier car (the fatality risk ratio) increased as a power function of the ratio of the mass of the heavier car to that of the lighter car (the mass ratio). The present study uses two sources of information to examine the relationship between these same quantities: first, calculations based on Newtonian mechanics, which show that when two cars crash head-on into each other, the ratio of their changes in speed (delta-v) is inversely proportional to mass ratio; second, National Accident Sampling System data, which show how delta-v affects driver injury risk. The study is performed for fatalities and severe injuries and for unbelted and belted drivers. Combining the two sources of information gives the result that fatality risk ratio increases as a power function of mass ratio, the same functional form found in the empirical study. Because the study is rooted in Newtonian mechanics, it clearly and directly identifies physical mechanisms involved and leads to the conclusion that mass, as such, causes large differences in driver injury and fatality risk when cars of unequal mass crash into each other.

Helmet effectiveness in preventing motorcycle driver and passenger fatalities

Helmet effectiveness in preventing fatalities to motorcycle drivers and passengers was determined by applying the double pair comparison method to the Fatal Accident Reporting System (FARS) data for 1975 through 1986. Motorcycles with a driver and a passenger, at least one of whom was killed, were used. In order to reduce as much as possible potentially confounding effects due to the dependence of survivability on sex and age, the analysis is confined to male drivers (there were insufficient female driver data), and to cases in which the driver and passenger age do not differ by more than three years. Motorcycle helmet effectiveness estimates are found to be relatively unaffected by performing the analyses in a number of ways different from that indicated above. It was found that helmets are (28 +/- 8)% effective in preventing fatalities to motorcycle riders (the error is one standard error), the effectiveness being similar for male and female passengers, and similar for drivers and passengers. An additional result found was that the fatality risk in the driver seat exceeds that in the passenger seat by (26 +/- 2)%. The 28% effectiveness found generates calculated fatality increases from repeal of mandatory helmet-wearing laws that are compatible with observed increases.

Driver Behavior Effects on Fuel Consumption in Urban Driving

Data are examined from two published experimental studies in which drivers were instructed to drive other than normally, with the traffic: for example, to minimize fuel consumption or to minimize trip time. It is found that, in general, for each 1% increase (decrease) in trip time compared to driving normally with the traffic, the fuel consumption increases (decreases) by approximately 1.1%. However, expert drivers can save fuel without increasing trip time by skillfully adjusting their speed to avoid stops at traffic signals. It is shown that the formal problem to be solved to minimize fuel consumption on an urban trip is so complicated that even a “perfect” fuel economy meter would not enable a driver to achieve this minimum.