Nikiforos Stamatiadis

Trends in highway safety: effects of an aging population on accident propensity

Aging of the United States population has a potentially adverse effect on highway safety. A key question is whether the known deterioration of driving skills with aging will be compensated by other factors, especially improved learning and attitudinal experiences of more recent cohorts. We investigate effects of driver age, cohort, and gender on accident propensity and evaluate confounding effects of year, location, and lighting. The accident propensity of different groups of drivers is measured, using a database of two-vehicle accidents, by the ratio of the number of at-fault drivers of a specific group to the corresponding number of not-at-fault drivers. Logistic regression modelling determines the statistical significance of the findings. The analysis reveals the following statistically significant effects: (a) middle-aged drivers are safer than younger drivers who, in turn, are safer than older drivers; (b) female drivers are safer on average than male drivers; (c) younger female drivers are safer than younger male drivers; (d) older male drivers are safer than older female drivers; (e) more recent cohorts of older drivers are safer than more distant cohorts; and (f) more distant cohorts of younger drivers are safer than more recent cohorts. We conclude that driver cohort provides a plausible explanation for many of the measurable, time-related accident trends that are observed. Accordingly, older drivers will be safer in the future because they will have acquired basic driving skills and attitudes in increasingly more automobile-dominated times. At the same time, the cohort effect appears small relative to other time related effects, notably aging, and older drivers will continue to be a high-risk component of the driving population and to require special consideration in driver education and licensing and in highway design and operations.

Quasi-induced exposure: methodology and insight

Even though the numerator in accident rates can be accurately determined nowadays, the denominator of these rates is an item of discussion and debate within the highway safety community. A critical examination of an induced exposure technique, based on the non-responsible driver/vehicle of a two-vehicle accident (quasi-induced exposure), is presented here. Differences in exposure for a series of accident location and time combinations are investigated, the assumption of similarities between drivers of single-vehicle accidents and the responsible driver of multiple-vehicle accidents is refuted, and the use of the non-responsible driver as a measure of exposure is tested using vehicle classification data. The results of the analyses reveal the following: (1) accident exposure is different for different location and time combinations: (2) induced exposure estimates provide an accurate reflection of exposure to multiple-vehicle accidents; (3) induced exposure estimates are acceptable surrogates for vehicle miles of travel when estimates are made for conditions during which the mix of road users is fairly constant; and (4) the propensity for involvement in single-vehicle accidents is generally different than that in multiple-vehicle accidents for a given class of road users. We concluded that the quasi-induced exposure is a powerful technique for measuring relative exposure of drivers or vehicles when real exposure data are missing.

Development of Accident Reduction Factors

The objective of this project is to develop accident reduction factors associated with various types of highway safety improvements. These factors will be used in the cost-optimization procedure to rank safety improvements. The basis of the accident reduction factors developed in this study is a survey of states and a review of literature. The recommended reduction factors are presented in a table which lists the percent reduction in all accidents or specific types of accidents for given types of improvements.

Impact of passengers on young driver safety

An analysis of young driver accidents uses the quasi-induced exposure technique to disaggregate accidents by different characteristics or circumstances while making estimates of the relative travel exposure for those different circumstances. Relationships are developed to investigate the effect of passengers on young driver (age 16 to 20) accident propensity. Three passenger groupings are used: solo (no passengers), peer, and adult or child. The database contains 77,312 accidents involving young drivers in Kentucky in 1994, 1995, and 1996. The passenger-group variable was found to have a significant effect on young driver accident propensities. Young drivers have the lowest propensity to cause single- or two-vehicle accidents when traveling with either adults and/or children. They have an increased propensity for causing single-vehicle accidents when traveling with peers. The relative accident involvement ratios for both single- and two-vehicle accidents for young drivers traveling alone were close to 1.0, suggesting that they were at fault approximately the same percentage of the time that they were not at fault. Seven variables were considered for possible interaction with passenger group: driver gender, total occupant gender, time of week, time of day, highway type, vehicle age, and safety-restraint usage.

PROBLEM DRIVING MANEUVERS OF ELDERLY DRIVERS

Older drivers, who are the fastest growing segment of the U.S. population, experience high crash rates. An analysis was performed to evaluate potential problem maneuvers that may lead to higher crash involvement. Left turns against oncoming traffic, gap acceptance for crossing non-limited-access highways, and high-speed lane changes on limited-access highways are identified as such maneuvers. Older and younger driver accident propensities are measured, using Kentucky crash data. The findings of the analysis show that older drivers are more likely to be involved in crashes related with these maneuvers compared with younger drivers; older male drivers are safer than older female drivers in left-turn crashes and gap acceptance–related crashes, and having a passenger beside the older drivers makes for a safer driving environment. Potential countermeasures aiming to reduce the accident rates of older drivers are discussed.

Gender Effect on the Accident Patterns of Elderly Drivers

Increased suburbanization, a greater dependency on the automobile, the continuous growth of the elderly population, and a significant increase in the number of elderly female drivers have created the need for research to investigate gender distinctions in the accident patterns of elderly drivers. Differences in accident patterns between younger and older drivers in this investigation are attributed partially to aging-related diminished abilities and partially to differences in drivership and licensure levels between the past and the present. This study analyzes and compares the accident patterns of elderly males and females. It shows that females aged 65 and over have higher accident involvement rates than elderly males. Accident exposure data indicate a similar exposure for elderly males and females making the higher rates for females more significant. In contrast, the accident patterns of males and females under 65 are similar, suggesting that gender-related differences may disappear in the future.

Causal Factors for Accidents on Southeastern Low-Volume Rural Roads

Low-volume roads comprise a significant portion of the rural roadway network. Because of documented higher crash frequencies and more severe injuries on such roads, it is necessary to further examine causal factors of these crashes and to determine if crash characteristics follow the patterns of other highways. The quasi-induced exposure technique is used to analyze the relationship of driver, roadway, and environmental factors involved in crashes on low-volume roads. Crashes from Kentucky and North Carolina for 1993–1995 are used. The results show that (a) low-volume roads present similar crash trends as those observed on other roads; (b) drivers under the age of 25 and drivers over the age of 65 have higher crash propensities than middle-aged drivers; (c) female drivers are safer on average than male drivers; (d) young drivers (under the age of 25) experience more single-vehicle crashes and drivers over 65 are more likely to be involved in two-vehicle crashes; (e) drivers of older vehicles have higher two-vehicle crash propensities on low-volume roads than drivers of newer vehicles; (f) in single-vehicle crashes, drivers of older vehicles are more likely to have a serious injury than drivers of newer vehicles; and (g) large trucks have the highest two-vehicle crash propensity on low-volume roads, followed by sedans, pick-up trucks, vans, and station wagons.

Crash rates and traffic maneuvers of younger drivers

Although the population of younger drivers has decreased over recent decades, their crash rates have increased. Research has associated their higher crash rates with societal influences and youthful behavior. Research was conducted to identify the specific driving maneuvers of which unsuccessful undertaking results in specific types of crashes involving younger drivers. Four types of crashes were identified as the most prominent for young drivers: intersection, rear end, passing, and single vehicle. The analysis was performed by examining the Kentucky crash database for the 1994-1996 period by using the quasi-induced exposure method. The results showed that for all crashes, there is a general trend of decreasing involvement with increasing age, which indicates that these drivers’ inexperience is the largest single contributor to their increased crash rates. Of significance is that for all crashes, a dramatic decrease of involvement after the first year of driving between the ages of 16 and 17 is observed. This may be indicative of a steep learning curve in the first years of driving regarding the ability to control a vehicle. Therefore, little can be done to improve this phenomenon. Increasing awareness among young drivers about these issues and their likely crash involvement appears to be the only viable approach. However, preliminary efforts from the graduated license program show that some trends have been reduced, indicating a possible influence on the crash rates of young drivers.

Impact of Shoulder Width and Median Width on Safety

This report contains the findings of research performed to quantify the safety and operational impacts of design element trade-offs and their associated risks. The report details the research performed and includes specific recommended crash prediction models and Accident Modification Factors (AMFs) for shoulder width and median width on rural four-lane roads. Thus, the report will be of immediate interest to engineers in state highway agencies responsible for geometric design and traffic operations and safety.

GUIDELINES FOR LEFT-TURN PHASING TREATMENT

The safety of left-turning vehicles at signalized intersections is a concern of the safety community, and guidelines for the installation of left-turn phasing have been developed. The development of guidelines to be considered when determining whether left-turn phasing should be used and the type of phasing that would be appropriate is presented. The guidelines are based on accident data collected for 408 approaches in Kentucky and delay data simulated for a variety of intersection characteristics. The recommendations made take into account many variables, including left-turn volumes, accident rates, product of opposing and left-turn volumes, and left-turn delays. A two-step approach is considered in the decision whether to install left-turn phasing: determine whether protection is needed and then determine the type of protection. The selection of the appropriate left-turn treatment is a process in which trade-offs between safety and delays may be required. Moreover, the use of single rules is usually not adequate to determine the appropriate left-turn treatment; a variety of other factors must be considered.