Relative Crash Risk Research Articles

Effects of winter precipitation on automobile collisions, injuries, and fatalities in the United States

To better understand the links between winter precipitation (snow, sleet, and freezing rain) and travel risk, data on weather conditions and vehicle crashes, injuries and fatalities are gathered for 13 U.S. cities. A matched pair analysis is used to construct event-control pairs to determine the relative risk of crash, injury, and fatality. Winter precipitation is associated with a 19% increase in traffic crashes and a 13% increase in injuries compared to dry conditions. The type of winter precipitation (snowfall vs. freezing rain, ice pellets, or sleet) had no significant impact on the relative risk of crash. The relative risk of crash was significantly higher during the evening (1800–2359 local time) than during other times of the day. More intense precipitation led to increased relative risk of crash and injury compared to less intense precipitation. Relative risk of crash, injury, or fatality was not significantly higher during the first three winter precipitation events of the year as compared to subsequent events. The relative risk of both winter precipitation crash and injury showed no significant trend during the 1998–2008 period. Sensitivity of U.S. cities to winter precipitation varies from city to city in a manner that is not easily explained. Future research will be required to determine which safety interventions are most effective in each city and revise or expand safety programs appropriately.

The relative risk of involvement in fatal crashes as a function of race/ethnicity and blood alcohol concentration

IntroductionThe literature presents a puzzling picture of Latinos being overrepresented in alcohol-related crashes, but not in noncrash drinking and driving. This report examines if, like other demographic variables in which some groups are at a higher crash risk than others (e.g., young drivers), different racial/ethnic groups face different crash risks. MethodThis study compares blood-alcohol information from the 2006–2007 U.S. Fatality Analysis Reporting System (FARS) with control data from the 2007 U.S. National Roadside Survey. Logistic regression, including a dual interaction between BAC and race/ethnicity, was used to estimate crash risk at different BAC levels. ResultsIt was found that, although Hispanic and African-American drivers were less likely to be involved in single-vehicle crashes than their White counterparts, all drivers face similar BAC relative crash risk regardless of their group membership. The overrepresentation of Latino drivers in alcohol-related crashes could be explained by differences in patterns of consumption, driving exposure, lack of awareness of driving rules, and/or socioeconomics.

Quantifying Crash Risk under Inclement Weather with Radar Rainfall Data and Matched-Pair Method

Driving risk under inclement weather has long been considered higher than that under normal weather conditions. Among several weather variables, rainfall is recognized as a critical one that affects traffic crash occurrences. Heavy rainfall often results in reduced visibility, increased stopping distances, hydroplaning, and loss of control of the vehicle. The lack of an accurate, quantitative representation of rainfall effects at a fine resolution has been a problem in past highway safety research on adverse weather. This study used radar rainfall data from the National Weather Service's WSR-88D NEXRAD Doppler Radar system that provides a comprehensive data set with large spatial coverage, high spatial resolution (in the order of few kilometers), and continuous temporal sampling to quantify the relative crash risk with a matched-pair method. Four different types of highways were selected for analysis over a 4-year time period. Each highway was divided into segments based on a fine resolution of rainfall data. The results of our analysis have indicated a higher crash risk and a higher injury risk during rain. The magnitude of the risk varies depending upon the type of highway, location of the highway, time of day, crash severity, and crash characteristics. Unlike previous studies, this project was able to identify the risk variations by type of highway and by segment because of the fine resolution of the data. The knowledge acquired by this study can be used to identify effective crash countermeasures.

Does vehicle colour influence crash risk?

An often asked question regarding vehicle safety is whether vehicle colour has an influence on crash risk and if so, what is the differential in risk between the various colours of vehicles available. The major objective of this study was to assess the relationship between vehicle colour and crash risk through the analysis of real crash outcomes described in Police reported crash data. The study employed induced exposure methods utilising single vehicle crashes as the comparison crash type. It estimated the crash risk associated with each vehicle colour relative to a reference colour which was chosen to be white. Results of the analysis identified a clear statistically significant relationship between vehicle colour and crash risk. Compared to white vehicles, a number of colours were associated with higher crash risk. These colours were black, blue, grey, green, red and silver. The association between vehicle colour and crash risk was strongest during daylight hours where relative crash risks were higher for the colours listed compared to white by up to around 10%. No colour had a statistically significantly lower crash risk than white although crash risks for a number of other colours were not statistically significantly different from white.

Road Crash Trends for Young Drivers in New South Wales, Australia, from 1997 to 2007

Objectives: The objective of this article was to explore overall crash and injury trends over the past decade for young drivers residing in New South Wales (NSW), Australia, including gender and age disparities. Methods: Passenger vehicle crashes for drivers aged 17–25 occurring during 1997–2007 were extracted from the state crash database to calculate crash rates (per licensed driver). Generalized linear models were used to examine crash trends over time by severity of driver injury, adjusting for age, gender, rurality of residence, and socioeconomic status. Yearly adjusted relative risks of crash by gender and by age group were also examined over the study period. Results: Young driver noninjury and fatality rates significantly decreased by an average of 4 percent (95% CI: 4–5) and 5 percent (95% CI: 0–9) respectively each year from 1997 to 2007. Young driver injury rates significantly increased by about 12 percent (95% CI: 9–14) to the year 2001 and then significantly decreased. The relative risk of crash (regardless of driver injury) for males compared to females significantly decreased over time. Compared to drivers aged 21–25, drivers aged 17 and particularly 18- to 20-year-olds had significantly and consistently higher crash risks across the study period. Conclusions: Overall, there has been a significant decline in young driver crashes in NSW over the last decade. Regardless of injury severity, males' risk of crash has reduced more than female young drivers, but drivers aged 17 continue to be at higher risk. These findings provide feedback on potential road safety successes and areas needing specific interventions for future improvements.

The Role of Reduced Fitness to Drive Due to Medical Impairments in Explaining Crashes Involving Older Drivers

Background. Medical conditions and associated impairments are known to be more prevalent with aging and can potentially impact the function and crash risk of older drivers. Objectives. To evaluate the impact of specific medical conditions and associated impairments on older driver crash risk. Methods. A search identified reports and peer-reviewed publications evaluating the risk for medical conditions and associated crash risk. Medical conditions associated with older persons were reviewed to determine the associated relative risk of crash. Results. The review identified three recent comprehensive reviews of medical conditions or chronic illnesses and crash risk: Dobbs (2005); Vaa (2003); Charlton et al. (2004). Comparison of the reviews reveals a relatively high agreement where medical conditions considered to be at slightly to moderately increased relative risk of crash include alcohol abuse and dependence, cardiovascular disease, cerebrovascular disease/TBI, depression, dementia, diabetes mellitus, epilepsy, use of certain medications, musculoskeletal disorders, schizophrenia, obstructive sleep apnea, and vision disorders. However, determining fitness to drive at the individual level based on diagnosis has significant limitations related to factors such as multiple medical conditions as well as varying severity of disease and associated functional impairments. Medical conditions that may affect driving can serve as “red flags” to assist health care professionals and driving administrators to identify drivers who may need further evaluation. Conclusions. Medical conditions overall, do impact the fitness to drive of older drivers; however, the crash risk tends to be only slightly to moderately increased. The conditions can serve as potential warnings for reduced fitness to drive, but many persons with these medical conditions would still be considered safe to continue driving.

Characteristics of unprotected left-turn accidents at signalised intersections

At signalized intersections, a major source of conflict is unprotected left-turning traffic. Relative crash risk is investigated by this study for frequently occurring unprotected left-turn accidents at signalized intersections. To perform this traffic safety analysis, quasi-induced exposure technique is used, based on the 1999-2001 Florida Accident Database. Numerous factors are shown by statistical tests to be associated with unprotected left-turn accidents, including type of vehicle, residence code, physical defect, alcohol use, driver gender, driver age, posted speed limit, weather, lighting conditions, divided/undivided highway, and highway lane numbers.

Driving Under the (Cellular) Influence: The Link between Cell Phone Use and Vehicle Crashes

The link between cell phone use while driving and crash risk has in recent years become an area of active research. The most notable of the over 125 studies has concluded that cell phones produce a four-fold increase in relative crash risk--comparable to that produced by illicit levels of alcohol. In response, policy makers in fourteen states have either partially or fully restricted driver cell phone use. We investigate the causal link between cellular usage and crash rates by exploiting a natural experiment induced by a popular feature of cell phone plans in recent years--the discontinuity in marginal pricing at 9 pm on weekdays when plans transition from peak to off-peak pricing. We first document a jump in call volume of about 20-30% at peak to off-peak switching times for two large samples of callers from 2000-2001 and 2005. Using a double difference estimator which uses the era prior to price switching as a control (as well as weekends as a second control), we find no evidence for a rise in crashes after 9 pm on weekdays from 2002-2005. The 95% CI of the estimates rules out any increase in all crashes larger than .9% and any increase larger than 2.4% for fatal crashes. These estimates are at odds with the crash risks implied by the existing research. We confirm our results with three additional empirical approaches--we compare trends in cell phone ownership and crashes across areas of contiguous economic activity over time, investigate whether differences in urban versus rural crash rates mirror identified gaps in urban-rural cellular ownership, and finally estimate the impact of legislation banning driver cell phone use on crash rates. None of the additional analyses produces evidence for a positive link between cellular use and vehicle crashes.

The relationship between blood alcohol concentration (BAC), age, and crash risk

Problem The role of age (youth and driving inexperience) and alcohol as major risk factors in traffic crash causation has been firmly established by numerous studies over the past 50 years. Less well established is how the two variables interrelate to influence crash risk. Some investigations have hypothesized an interactive or synergistic effect in which young drivers with less experience and a greater tendency to take risks are more adversely affected at lower blood alcohol concentrations (BACs) than are older drivers. The evidence for this hypothesis is mixed. Resolution of this issue has important implication for developing countermeasures directed at the young driver crash problem. Method Case control data previously collected in Long Beach and Fort Lauderdale were reanalyzed using a more sensitive method for detecting interaction effects than used in the original analysis. A conditional logistic regression analyses found a highly significant age × BAC interaction (P < .0001) involving differences between drivers under 21 and those 21 and older. Discussion The results clearly indicate that positive BACs in drivers under 21 are associated with higher relative crash risks than would be predicted from the additive effect of BAC and age. It is likely that two mechanisms are operating to cause the interaction. First, it seems likely that the crash avoidance skill of young novice drivers would be more adversely affected by alcohol due to their driving inexperience, immaturity, and less experience with alcohol. Second, drivers under 21 who choose to drink and to drive after drinking probably have pre-existing characteristics that predisposed them to risk taking and crash involvement apart from any increased vulnerability to alcohol impairment. Impact on Industry The results support increased enforcement of zero-tolerance BAC laws for minors.

Bayesian Approach Based on Geographic Information System to Identify Hazardous Roadway Segments for Traffic Crashes

This paper presents a Bayesian approach based on a geographic information system (GIS) for areawide identification of hazardous roadway segments for traffic crashes. The conventional observed relative crash risks of roadway segments are based on the observed crash count data and often feature very small or very large outlying relative risks in segments with low traffic volumes or short segment lengths; thus the risk maps based on observed crash risks show a high uncertainty. The Bayesian approach can incorporate both the variability in the observed risks and spatial autocorrelation and use the typical Bayesian “borrow of strength” from adjacent segments to filter out the uncertainties, smooth the relative risks, and accurately identify and rank roadway segments with potentially high relative risks for crashes. To capture the real safety indications better, this paper also disaggregates different road segment types and utilizes GIS to analyze the crash data and visualize the spatial relative crash risks in three-dimensional views. Houston, Texas, is chosen as the study area, and three crash data sets (all KAB crashes: crashes with fatality [K], incapacitating injury [A], and nonincapacitating injury [B]; KAB crashes under unsafe speeding; and KAB crashes under the influence of alcohol) in year 2000 are analyzed. Results demonstrate that the approach is effective in spatially smoothing the relative crash risks and eliminating the instability of estimates, while maintaining real safety trends, especially for data with a small sample size and large variance. The result provides a sound basis for implementing preventive actions to reduce crashes on high-risk segments.

A GIS-based Bayesian approach for analyzing spatial–temporal patterns of intra-city motor vehicle crashes

This paper develops a GIS-based Bayesian approach for intra-city motor vehicle crash analysis. Five-year crash data for Harris County (primarily the City of Houston), Texas are analyzed using a geographic information system (GIS), and spatial–temporal patterns of relative crash risks are identified based on a Bayesian approach. This approach is used to identify and rank roadway segments with potentially high risks for crashes so that preventive actions can be taken to reduce the risks in these segments. Results demonstrate the approach is useful in estimating the relative crash risks, eliminating the instability of estimates while maintaining overall safety trends. The 3-D posterior risk maps show risky roadway segments where safety improvements need to be implemented. Results of GIS-based Bayesian mapping are also useful for travelers to choose relatively safer routes.

Ageing drivers: Storm in a teacup?

The purpose of this paper is to address two important issues regarding ageing drivers. First, there is a presumption in the road safety arena that the expected increase in the number of ageing drivers on the roads will lead to an increase in crashes. Second, despite extensive research on ageing drivers, especially on their increased vulnerability and reduced driving abilities, the most widely recommended road safety strategy relates to the control of their driving licenses. This presumption and the associated policy recommendation are based mainly on the higher relative crash risks associated with ageing drivers compared to their younger counterparts. This study, however, argues that the average crash risks obtained in previous studies provide only indirect information concerning these issues and any decision based on these results may produce unexpected outcomes. An analytical framework to examine the marginal effect of changing the driver mix on the roads is proposed and a simple empirical model is estimated as an illustration. In contrast to previous studies, we found that increasing the number of licenses issued to ageing drivers had only an insignificant impact on the number of fatal crashes on the roads, suggesting that the concern over ageing drivers may be a storm in a teacup.

Bayesian Identification of High-Risk Intersections for Older Drivers via Gibbs Sampling

Hierarchical Bayes methods are combined with an induced exposure model in order to identify intersections where the crash risk for a given driver subgroup is relatively higher than that for some other group. The necessary computations are carried out using Gibbs sampling, producing point and interval estimates of relative crash risk for the specified driver group at each site in a sample. The method is applied to data from 102 signalized intersections, and 10 were identified as showing high risk for older drivers. Left-turn crashes tended to predominate at these 10, whereas rear-end crashes were most common at geographically similar intersections not identified as showing high risk to older drivers.

Antilock Brake Systems and Risk of Different Types of Crashes in Traffic

While antilock braking systems (ABS) have been convincingly demonstrated to enhance test track braking performance, their effect on crash risk in actual driving remains less clear. This paper examines how ABS influences crash risk using mainly two published studies which used police-reported crashes. The published findings are augmented by including new data and additional results. All the work is based on seven General Motors (GM) passenger vehicles having ABS as standard equipment for 1992 models but not available for 1991 models. The ratio of crashes under an adverse condition (say, when the pavement is wet) to under a normal condition (say, when the pavement is dry) is compared for ABS and non-ABS vehicles. After correcting for such factors as model year effects not linked to ABS, the following associations between ABS and crash risk were found by averaging data from the five states Texas, Missouri, North Carolina, Pennsylvania and Indiana (the errors are one standard error): a (10 ± 3)% relative lower crash risk on wet roads compared to the corresponding comparison on dry roads; a (22 ± 11)% lower risk of a pedestrian crash compared to the risk of a non-pedestrian crash; a (39 ± 16)% increase in rollover crash risk compared to the risk of a non-rollover crash. Data from the same Ave states were used to examine two-vehicle rear-end collisions. Using the assumption that side-impact crashes estimate exposure, it was found that for wet roads ABS reduces the risk of crashing into a lead vehicle by (32 ± 8)%, but increases the risk of being struck in the rear by (30 ± 14)%. The results from this study and from all available reported studies are summarized in tabular form.

Fatal crash risk for older drivers at intersections

Risk of fatal crash involvement was calculated for older drivers relative to drivers aged 40–49 in the United States during the years 1994–1995. The results indicated that drivers ages 65–69 were 2.26 times more at risk for multiple-vehicle involvements at intersections compared with 1.29 times more at risk in all other situations. The comparable figures for drivers aged 85 and older were 10.62 for multiple-vehicle involvements at intersections compared with 3.74 for all other situations. The relative crash risk was particularly high for older drivers at uncontrolled and stop sign-controlled locations; when traveling straight or when just starting to enter the intersection; and when the specific behavioral error in the crash was failure to yield. Countermeasures will likely involve reducing or simplifying the need to detect and evaluate moving traffic coming from the left and right when at intersections. This can be accomplished by traffic signals with protected left turns, four-way stop signs, and one-way streets. Whereas such devices involve significant cost in terms of dollars and travel delay, their cost-effectiveness may have to be revisited as the United States population continues to age.

Dementia and driving: an attempt at consensus.

The number of older drivers in Sweden will be rapidly increasing during the next decades. A possible relationship exists between the increased relative crash risk of older drivers and the prevalence of age-related diseases such as dementia. However, a clear-cut policy for evaluating driving competence in demented persons is still lacking. In recognition of this fact, the Swedish National Road Administration invited a group of researchers to formulate a consensus on the issue of driving and dementia. This consensus document is aimed at providing primary care physicians with practical advice concerning the assessment of cognitive status in relation to driving. Suggestions are based on a review of existing research and discuss the use of general and driving-specific sources of information available to the physician. Consensus was reached on the statement that a diagnosis of moderate to severe dementia precludes driving and that certain individuals with mild dementia should be considered for a specialized assessment of their driving competence.

Youth, alcohol and relative risk of crash involvement

Deaths and injuries due to road-crash involvement are a major health and safety problem, especially among youth. Numerous factors can account for the overrepresentation of young drivers in road crashes and one of these—alcohol—has received renewed attention. This paper examines evidence pertaining to drinking and driving among youth to determine the extent to which alcohol has special significance for crashes involving young drivers. Three sets of studies are reviewed: those providing data on the extent of drinking and drinking-driving among youth, those that examine alcohol use among youth involved in road crashes and those that estimate the relative risk of a road crash for young drinking drivers. Findings show that frequent and heavy alcohol consumption among teenagers and young adults is not unusual, although they are less likely than older age groups to drive after drinking. Nonetheless, those young people who drive after drinking have a greater risk of crash involvement than older drinking drivers at all blood alcohol concentrations. One explanation of this finding assumes that young people are “inexperienced” with drinking and with driving. Limited behavioural research done to date supports some but not all assumptions implicit in this reasoning. There is also evidence in favour of an alternative hypothesis, namely, that the comparatively higher crash-risk among young drinking drivers is attributable to a subset of this group, those that engage in risky driving behaviour and who also happen to use alcohol. Further investigation into the personal and social characteristics of young people who drive after drinking would provide a sounder empirical basis for policies and programmes to reduce crash involvement among youth.

An evaluation of responsibility analysis for assessing alcohol and drug crash effects

This paper evaluates judgments of driver crash responsibility to estimate alcohol and drug impairment effects when exposure data are unavailable to calculate crash risks. Previous studies using responsibility judgments provided some evidence that responsibility is related to BAC. Other studies, some inferring responsibility, indicated a relation between responsibility and relative crash risk. Data are presented showing that responsibility judgments with a rating scale have high interrater reliability, and systematic relations with BAC suggest some validity in the ratings. A method is demonstrated for estimating relative crash risk from responsibility judgments with accident data, and the limitations of responsibility analysis are discussed. While alcohol and drug impairment effects are best determined with relative crash risks determined from accident and exposure data. responsibility analysis may provide useful indications in the absence of exposure data.