The driver's role in fatal two-car crashes: a paired "case-control" study.

This population-based study examines drivers' characteristics associated with driving errors that resulted in fatal motor vehicle crashes. Routinely collected data from the Fatal Accident Reporting System were used to assess whether a driver initiated the crash (case) or was passively involved (control) in 6,506 two-car collisions (81% of 7,993 eligible events). A paired comparison of cases and controls avoided confounding by environmental factors, exposure to traffic, and differences in case fatality. The strongest predictor of crash initiation is alcohol (odds ratio (OR) = 11.5; 95% confidence interval (CI) 9.57-13.9). Odds ratios are elevated even at the lowest blood alcohol concentration levels and increase dramatically as alcohol levels rise. Drivers aged 40-49 years are least likely to initiate crashes; odds ratios rise in a U-shaped manner to 3.35 in teenagers (95% CI 2.72-4.13) and to 22.1 in drivers over 80 years (95% CI 14.2-34.5). Other risk factors for initiating a fatal crash are the following: not wearing a seat belt (OR = 1.54; 95% CI 1.35-1.75), driving without a valid driver's license (OR = 2.16; 95% CI 1.72-2.73), and having had a crash within the last year (OR = 1.21; 95% CI 1.07-1.38). Driving errors leading to fatal crashes do not occur at random, but are associated with specific driver characteristics. The risk factors for crash initiation among crash-involved drivers are similar to risk factors for crash involvement found in other studies. These findings suggest that driving errors often explain high rates of crash involvement, invite further use of crash initiation in traffic injury research, and underscore the value of population-based registries for analytic epidemiology.     

Relations among chronic medical conditions, medications, and automobile crashes in the elderly: a population-based case-control study

Older drivers have elevated crash rates and are more likely to be injured or die if they have a crash. Medical conditions and medications have been hypothesized as determinants of crash involvement. This population-based case-control study sought to identify medical conditions and medications associated with risk of at-fault crashes among older drivers. A total of 901 drivers aged 65 years and older were selected in 1996 from Alabama Department of Public Safety driving records: 244 at-fault drivers involved in crashes; 182 not at-fault drivers involved in crashes; and 475 drivers not involved in crashes were enrolled. Information on demographic factors, chronic medical conditions, medications, driving habits, visual function, and cognitive status was collected. Older drivers with heart disease (odds ratio (OR) = 1.5, 95% confidence interval (CI): 1.0, 2.2) or stroke (OR = 1.9, 95% CI: 0.9, 3.9) were more likely to be involved in at-fault automobile crashes. Arthritis was also associated with an increased risk among females (OR =1.8, 95% CI: 1.1, 2.9). Use of nonsteroidal antiinflammatory drugs (OR = 1.7, 95% CI 1.0, 2.6), angiotensin converting enzyme inhibitors (OR = 1.6, 95 CI: 1.0, 2.7), and anticoagulants (OR = 2.6, 95% CI: 1.0, 73) was associated with an increased risk of at-fault involvement in crashes. Benzodiazepine use (OR = 5.2, 95% CI: 0.9, 30.0) was also associated with an increased risk. Calcium channel blockers (OR = 0.5, 95% CI: 0.2, 0.9) and vasodilators (OR = 0.3, 95% CI: 0.1, 1.0) were associated with a reduced risk of crash involvement. The identification of medical conditions and medications associated with risk of crashes is important for enhancing the safety and mobility of older drivers.     

Characteristics of Crashes With Farm Equipment That Increase Potential for Injury

CONTEXT: Crash fatality and injury rates are higher on rural roadways than other roadway types. Although slow-moving farm vehicles and equipment are risk factors on rural roads, little is known about the characteristics of crashes with farm vehicles/equipment. PURPOSE: To describe crashes and injuries for the drivers of farm vehicles/equipment and non-farm vehicles involved in an injury crash. Passengers are not included in this analysis. METHODS: Injury crashes were included that involved a farm vehicle/equipment and at least one non-farm vehicle reported in Iowa Department of Transportation crash data from 1995 to 2004. Odds ratios were calculated through logistic regression to identify increased odds for injury among drivers of non-farm vehicles and farm vehicles/equipment. We examined frequently occurring crash characteristics to identify crash scenarios leading to the highest odds for injury. FINDINGS: Non-farm vehicle drivers were 5.23 times more likely to be injured than farm vehicle/equipment drivers (95% CI = 4.12-6.46). The absence of restraint use was a significant predictor of injury for both farm vehicle/equipment drivers (OR = 2.85; 95% CI = 1.14-7.13) and non-farm vehicle drivers (OR = 2.53; 95% CI = 1.54-4.15). Crash characteristics increasing the odds of injury for non-farm vehicle drivers included speeding, passing the farm vehicle/equipment, driving on a county road, having a frontal impact collision, and crashing in darkness. Ejection was the strongest predictor of injury for the farm vehicle/equipment driver. CONCLUSION: Non-farm vehicle drivers were much more likely to be injured than farm vehicle/equipment drivers, suggesting that farm vehicle/equipment crash prevention should be a priority for all rural road users. Prevention strategies that reduce motor vehicle speed, assist in safe passing, increase seat belt use, and increase conspicuousness of the farm vehicle/equipment are suggested.     

Alcohol, psychoactive substances and non-fatal road traffic accidents - a case-control study

ABSTRACT: BACKGROUND: The prevalence of alcohol and other psychoactive substances is high in biological specimens from injured drivers, while the prevalence of these psychoactive substances in samples from drivers in normal traffic is low. The aim of this study was to compare the prevalence of alcohol and psychoactive substances in drivers admitted to hospital for treatment of injuries after road traffic accidents with that in drivers in normal traffic, and calculate risk estimates for the substances, and combinations of substances found in both groups. METHODS: Injured drivers were recruited in the hospital emergency department and drivers in normal conditions were taken from the hospital catchment area in roadside tests of moving traffic. Substances found in blood samples from injured drivers and oral fluid samples from drivers in moving traffic were compared using equivalent cut off concentrations, and risk estimates were calculated using logistic regression analyses. RESULTS: In 21.9% of the injured drivers, substances were found: most commonly alcohol (11.5%) and stimulants eg. cocaine or amphetamines (9.4%). This compares to 3.2% of drivers in normal traffic where the most commonly found substances were z-hypnotics (0.9%) and benzodiazepines (0.8%). The greatest increase in risk of being injured was for alcohol combined with any other substance (OR: 231.9, 95% CI: 33.3- 1615.4, p < 0.001), for more than three psychoactive substances (OR: 38.9, 95% CI: 8.2- 185.0, p < 0.001) and for alcohol alone (OR: 36.1, 95% CI: 13.2- 98.6, p < 0.001). Single use of non-alcohol substances was not associated with increased accident risk. CONCLUSION: The prevalence of psychoactive substances was higher among injured drivers than drivers in normal moving traffic. The risk of accident is greatly increased among drivers who tested positive for alcohol, in particular, those who had also ingested one or more psychoactive substances. Various preventive measures should be considered to curb the prevalence of driving under the influence of psychoactive substances as these drivers constitute a significant risk for other road users as well as themselves.     

Effects of snowfalls on motor vehicle collisions, injuries, and fatalities

OBJECTIVES: We estimated the effects of snowfalls on US traffic crash rates between 1975 and 2000. METHODS: We linked all recorded fatal crashes (1.4 million) for the 48 contiguous states from 1975 through 2000 to daily state weather data. For a subsample including 17 states during the 1990s, we also linked all recorded property-damage-only crashes (22.9 million) and nonfatal-injury crashes (13.5 million) to daily weather data. Employing negative binomial regressions, we investigated the effects of snowfall on crash counts. Fixed effects and other controls were included to address potential confounders. RESULTS: Snow days had fewer fatal crashes than dry days (incidence rate ratio [IRR] = 0.93; 95% confidence interval [CI] = 0.90, 0.97), but more nonfatal-injury crashes (IRR = 1.23; 95% CI = 1.18, 1.29) and property-damage-only crashes (IRR=1.45; 95% CI=1.38, 1.52). The first snowy day of the year was substantially more dangerous than other snow days in terms of fatalities (IRR = 1.14; 95% CI=1.08, 1.21), particularly for elderly drivers (IRR=1.34; 95% CI=1.23, 1.50). CONCLUSIONS: The toll of snow-related crashes is substantial. Our results may help estimate the potential benefits of safety innovations currently proposed by meteorology and traffic safety experts.     

Causal influence of car mass and size on driver fatality risk

OBJECTIVES: This study estimated how adding mass, in the form of a passenger, to a car crashing head-on into another car affects fatality risks to both drivers. The study distinguished the causal roles of mass and size. METHODS: Head-on crashes between 2 cars, one with a right-front passenger and the other with only a driver, were examined with Fatality Analysis Reporting System data. RESULTS: Adding a passenger to a car led to a 14.5% reduction in driver risk ratio (risk to one driver divided by risk to the other). To divide this effect between the individual drivers, the author developed equations that express each driver's risk as a function of causal contributions from the mass and size of both involved cars. Adding a passenger reduced a driver's frontal crash fatality risk by 7.5% but increased the risk to the other driver by 8.1%. CONCLUSIONS: The presence of a passenger reduces a driver's frontal crash fatality risk but increases the risk to the driver of the other car. The findings are applicable to some single-car crashes, in which the driver risk decrease is not offset by any increase in harm to others. When all cars carry the same additional cargo, total population risk is reduced.     

Prospective study of brief neuropsychological measures to assess crash risk in older primary care patients

BACKGROUND: Practicing primary care physicians often encounter the difficult clinical situation of evaluating the older driver. We wanted to investigate the relation between self-reported driving behavior, neuropsychological measures, and crash risk to inform the development of a test battery that could predict unsafe driving behavior and was feasible for use by primary care physicians. METHODS: This study was a prospective follow-up of 107 drivers aged 65 years and older recruited from a primary care setting in 1995. Tests of attention, visual information processing, spatial orientation, and general mental status were administered at baseline. At baseline and after 2 years of follow-up, patients were asked about their driving history using the driving questionnaire. Risk for reported crashes in the follow-up period was assessed in relation to baseline driving history and measures of cognition. RESULTS: Baseline self-reports of driving habits and attitudes were associated with an increased risk of reporting a crash after 2 years of follow-up (relative risk ratio = 5.31; 95% confidence interval [CI], 0.63, 44.63). In addition, baseline tests of attention, visual information processing, and spatial orientation were associated with an increased risk of reporting motor vehicle crash at follow-up. For example, respondents with poor performance on the Trail Making Test-part A, were almost four times more likely to report a crash at follow-up (risk ratio = 3.15; 95% CI, 0.76, 13.07). CONCLUSION: Although our conclusions are tempered by small sample size, this preliminary study suggests that brief cognitive tests and simple questions about driving habits warrant further investigation as indicators of crash risk with potential utility for assessing older drivers in primary care.     

Psychoactive Substance Use and the Risk of Motor Vehicle Crash Injuries in Southern Taiwan

The purpose of this study was to examine the association between psychoactive drug use and motor vehicle crash (MVC) injuries requiring hospitalization in southern Taiwan. A case–control study was conducted in southern Taiwan from January 2009 to December 2009. The cases included car or van drivers who were involved in MVCs and required hospitalization. Demographic and trauma-related data were collected from questionnaires and hospital and ambulance records. Urine and/or blood samples were collected on admission. The controls consisted of drivers who were randomly recruited while driving on public roads. Study subjects were interviewed and asked to provide urine samples. All blood and urine samples were tested for alcohol and a number of other legal and illegal drugs. Only those subjects who provided urine and/or blood specimens were included in the study. During the study period, 254 case patients and 254 control drivers were enrolled. The analysis showed an odds ratio (OR) of 3.41 (95% confidence intervals (95% CI), 1.76–6.70; p < 0.001) for persons taking benzodiazepines, and an OR of 3.50 (95% CI, 1.81–6.85; p < 0.001) for those taking alcohol (blood alcohol concentrations (BAC) ≥ 0.8 g/l) with regard to hospitalizations due to MVCs. For persons taking combinations of benzodiazepines and alcohol, the OR increased to 5.12 (95% CI: 1.77–15.91, p < 0.001). This study concluded that drug use among motor vehicle drivers increases the risk of MVCs that require hospitalization. From a public health perspective, the high risk ratios are concerning, and preventive measures are warranted.     

Obesity and risk for death due to motor vehicle crashes

OBJECTIVES: We examined the role of body mass index (BMI) and other factors in driver deaths within 30 days after motor vehicle crashes. METHODS: We collected data for 22 107 drivers aged 16 years and older who were involved in motor vehicle crashes from the Crashworthiness Data System of the National Automotive Sampling System (1997-2001). We used logistic regression and adjusted for confounding factors to analyze associations between BMI and driver fatality and the associations between BMI and gender, age, seatbelt use, type of collision, airbag deployment, and change in velocity during a crash. RESULTS: The fatality rate was 0.87% (95% confidence interval [CI]=0.50, 1.24) among men and 0.43% (95% CI=0.31, 0.56) among women involved as drivers in motor vehicle crashes. Risk for death increased significantly at both ends of the BMI continuum among men but not among women (P<.05). The association between BMI and male fatality increased significantly with a change in velocity and was modified by the type of collision, but it did not differ by age, seatbelt use, or airbag deployment. CONCLUSIONS: The increased risk for death due to motor vehicle crashes among obese men may have important implications for traffic safety and motor vehicle design.     

道路伤害的病例对照研究

目的：探讨道路伤害危险因素。方法：采用病例对照研究方法，于2001年11月至2002年8月收集沈阳市皇姑区发生机动车交通事故的事故组驾驶员406例，并同期在皇姑区内于随机时间、随机地点调查道路上正常行驶的对照组驾驶员438名。采用统一问卷、面询方法，调查内容包括驾驶员的一般情况，连续驾驶时间，事故／调查前睡眠状况，急、慢性困倦程度(采用Stanford和Epworth困倦量表测量)，饮酒，吸烟，驾驶安全态度和行为，车速，车辆状况等。结果：处于慢性困倦状态的驾驶员发生事故的危险性是非困倦状态驾驶员的1．98倍(OR=1．98，95％CI：1．26—3．12)，事故组驾驶员的困倦程度高于对照组，但差异无显著性(OR＝2．38，95％CI：0．89—6．31)。夜班或倒班发生事故的危险是常白班的2．09倍(OR＝2．09，95％CI：1．48—2．94)，酒后驾车发生事故的危险性是非酒后驾车的3．59倍(OR＝3．59，95％CI：1．13--11．39)，无人约束时会违章的驾驶员发生事故的危险性是不违章驾驶员的1．73倍(OR＝1．73，95％CI：1．22—2．46)。结论：慢性困倦、夜班或倒班、酒后驾车、违章等是道路伤害的危险因素，急性困倦可能是道路伤害的一个潜在危险因素。     

Fracture history and risk of breast and endometrial cancer

Fractures in postmenopausal women may serve as a surrogate measure of bone density, reflecting long-term lower estrogen levels, and lower estrogen levels appear to be inversely associated with breast and endometrial cancer. Breast cancer cases aged 50-79 years (n = 5,559) and endometrial cancer cases aged 40-79 years (n = 739) were enrolled in a US case-control study in 1992-1994 to evaluate the relation between fractures and risk of breast and endometrial cancer. Controls for the breast cancer analysis (n = 5,829) and the endometrial cancer analysis (n = 2,334) were randomly selected from population lists (driver's license and Medicare files). Information on fracture history and other risk factors was obtained by telephone interview. Compared with women without a fracture in the past 5 years, the odds ratios for women with a history of fracture were 0.80 (95% confidence interval (CI): 0.68, 0.94) for breast cancer and 0.59 (95% CI: 0.40, 0.89) for endometrial cancer. Height loss (> or =2.5 cm) and recent fracture history were associated with the lowest risk of breast cancer (odds ratio = 0.62, 95% CI: 0.46, 0.83) and endometrial cancer (odds ratio = 0.15, 95% CI: 0.05, 0.43). These data suggest that the endogenous hormonal factors associated with increased fracture risk are also related to decreased breast cancer risk and, more strongly, to endometrial cancer risk.     

The relation between multiple births and maternal risk of breast cancer.

Data from two case-control studies conducted in New York State during 1982-1986 were used to examine the relation between multiple births and the maternal risk of breast cancer. The cases were 2,561 women between 20 and 79 years of age with a diagnosis of primary breast cancer. Controls (n = 2,616) were selected from driver's license files and matched to cases by year of birth and county of residence. The odds ratio for any multiple birth was 0.94 (95% confidence interval (CI) 0.56-1.56) in women less than 55 years of age and 0.95 (95% CI 0.62-1.46) in women aged 55-79 years. A previous study had shown a multiple last birth to be protective against breast cancer in women less than 55 years of age (odds ratio (OR) = 0.60, 95% CI 0.43-0.85). A decreased risk of breast cancer was also observed for this age group in the present study, but the magnitude of the effect was not as strong and the confidence interval included unity (OR = 0.85, 95% CI 0.43-1.68). A logistic model that controlled for age at first pregnancy, number of live births, age, and county of residence increased the odds ratio to 0.97 for a multiple last birth. The current study does not support an association between multiple births and maternal risk of breast cancer.     

Fatal crashes among older drivers: decomposition of rates into contributing factors

This study selected US drivers aged 55 years or older who were involved in fatal crashes in 1990 and 1995 and explored factors that influenced their fatal crash involvement rate. The fatal crash involvement rate (risk of being involved in a fatal crash) can be thought of as the product of the crash fatality rate (risk of dying given a crash), the crash incidence density (risk of crash), and the exposure prevalence (amount of driving). Fatal crash involvement rates increased with age. The relative contributions of the crash incidence densities and exposure prevalences were greater than that of the crash fatality rates. The decomposition methodology was shown to be a useful method for investigating the potential benefit of crash prevention interventions aimed at different components of the fatal crash involvement rate.     

Availability of driver's license master lists for use in government-sponsored public health research

Although the percentage of US drivers with valid driver's licenses varies from state to state, it has historically been high enough to constitute a useful sampling frame for many public health purposes. Over the past decade, states have had to restrict access to this information to comply with the Driver's Privacy Protection Act (18 U.S.C. 2721-2725). In 2009 and 2010, the authors conducted a survey of all 50 states on the availability of master lists of licensed drivers to be used to contact citizens of each state for research purposes. A hypothetical situation requiring driver's license data was sent to each state's responsible government agency for review. In addition, the authors collected data on opt-out mechanisms available to drivers, costs to researchers, and additional state privacy policies pertaining to driver's license files. A total of 42 states (84%) responded; 16 (32%) states allowed access to data, 4 (8%) states were unable to respond to the hypothetical situation, and 22 (44%) states denied access to data. A total of 74,697,574 records were available from the 16 states providing driver's license data. Although the Driver's Privacy Protection Act has restricted access to data on licensed drivers, these data are still an available resource in many states.     

Weight change and risk of endometrial cancer

BACKGROUND: Obesity is an established risk factor for endometrial cancer. Less well understood is the role of weight gain and weight change in determining risk. METHODS: We analysed data from a population-based case-control study to evaluate the associations of body mass index (BMI), weight gain, and weight cycling with risk of endometrial cancer. Cases (n=740) under age 80 with a new diagnosis of endometrial cancer were identified from Wisconsin's cancer registry. Controls (n=2342) were randomly selected from driver's license lists and Medicare beneficiary files. Body size at three time points and other risk factor information were ascertained by interview in 1992-95. RESULTS: Endometrial cases were more likely than controls to be nulliparous, have early ages at menarche and late ages at menopause, be diabetic, smoke cigarettes, and use post-menopausal hormones. After adjustment for these factors, increasing BMI was associated with increased risk (P-trend<0.001); women in the top quartile of BMI (>29 kg/m2) had a 3-fold greater risk of endometrial cancer [95% confidence interval (95% CI) 2.4-4.2] compared with women in the lowest quartile (<23 kg/-m2). For each 5 kg weight gain, the odds ratio (OR) for endometrial cancer risk equalled 1.2 (95% CI 1.2-1.3). History of weight cycling modestly increased risk after adjustment for BMI and other factors (OR=1.3; 95% CI 1.0-1.6). In addition, women who reported sustained weight loss had a reduced risk of endometrial cancer (OR=0.7; 95% CI 0.6-0.9). CONCLUSIONS: These results suggest that weight gain and lack of weight stability are associated with risk of endometrial cancer.     

Minimum purchasing age for alcohol and traffic crash injuries among 15- to 19-year-olds in New Zealand

OBJECTIVES: In 1999, New Zealand lowered the minimum purchasing age for alcohol from 20 to 18 years. We tested the hypothesis that this increased traffic crash injuries among 15- to 19-year-olds. METHODS: Poisson regression was used to compute incidence rate ratios for the after to before incidence of alcohol-involved crashes and hospitalized injuries among 18- to 19-year-olds and 15- to 17-year-olds (20- to 24-year-olds were the reference). RESULTS: Among young men, the ratio of the alcohol-involved crash rate after the law change to the period before was 12% larger (95% confidence interval [CI]=1.00, 1.25) for 18- to 19-year-olds and 14% larger (95% CI=1.01, 1.30) for 15- to 17-year-olds, relative to 20- to 24-year-olds. Among young women, the equivalent ratios were 51% larger (95% CI=1.17, 1.94) for 18- to 19-year-olds and 24% larger (95% CI=0.96, 1.59) for 15- to 17-year-olds. A similar pattern was observed for hospitalized injuries. CONCLUSIONS: Significantly more alcohol-involved crashes occurred among 15-to 19-year-olds than would have occurred had the purchase age not been reduced to 18 years. The effect size for 18- to 19-year-olds is remarkable given the legal exceptions to the pre-1999 law and its poor enforcement.     

Motorcycle licensure, ownership, and injury crash involvement.

The interrelationships among motorcycle licensure, ownership, and injury crash involvement were investigated in a sample of 2,723 motorcycle drivers severely or fatally injured in California in 1985-86. Owners of motorcycles in such crashes ("driver-owners") were less likely to have valid licenses than a random sample of motorcycle owners who had not been in crashes (42 vs. 57 percent). Thirty-three percent of the crash-involved drivers had valid motorcycle driver's licenses; 39 percent were operating motorcycles they did not own ("driver-nonowners"). Driver-nonowners were less likely to be validly licensed than driver-owners (20 percent vs. 44 percent). The licensing rate of crash-involved driver-nonowners was 15 percent if the owner was also unlicensed. Rates of valid licensure were lowest among the youngest drivers. Virtually no crash-involved driver-nonowners under age 21 were licensed in cases in which the owner was also young and unlicensed.     

Marijuana use and motor vehicle crashes

Since 1996, 16 states and the District of Columbia in the United States have enacted legislation to decriminalize marijuana for medical use. Although marijuana is the most commonly detected nonalcohol drug in drivers, its role in crash causation remains unsettled. To assess the association between marijuana use and crash risk, the authors performed a meta-analysis of 9 epidemiologic studies published in English in the past 2 decades identified through a systematic search of bibliographic databases. Estimated odds ratios relating marijuana use to crash risk reported in these studies ranged from 0.85 to 7.16. Pooled analysis based on the random-effects model yielded a summary odds ratio of 2.66 (95% confidence interval: 2.07, 3.41). Analysis of individual studies indicated that the heightened risk of crash involvement associated with marijuana use persisted after adjustment for confounding variables and that the risk of crash involvement increased in a dose-response fashion with the concentration of 11-nor-9-carboxy-delta-9-tetrahydrocannabinol detected in the urine and the frequency of self-reported marijuana use. The results of this meta-analysis suggest that marijuana use by drivers is associated with a significantly increased risk of being involved in motor vehicle crashes.     

Role of alcohol and marijuana use in the initiation of fatal two-vehicle crashes

PurposeTo assess individual and joint effects of alcohol and marijuana on the initiation of fatal two-vehicle crashes.MethodsData on 14,742 culpable drivers (initiators) and 14,742 nonculpable drivers (noninitiators) involved in the same fatal two-vehicle crashes between 1993 and 2014 were obtained from the Fatality Analysis Reporting System. Multivariable conditional logistic regression models were used to assess the association of driver use of alcohol, marijuana, or both with fatal crash initiation with adjustment for demographic variables.ResultsInitiators were significantly more likely than non-initiators to test positive for alcohol (28.3% vs. 9.6%, P < .0001), marijuana (10.4% vs. 6.0%, P < .0001), and both substances (4.4% vs. 1.1%, P < .0001). Relative to drivers testing negative for both alcohol and marijuana, the adjusted odds ratios of fatal crash initiation were 5.37 (95% confidence interval [CI]: 4.88 to 5.92) for those testing positive for alcohol and negative for marijuana, 1.62 (95% CI: 1.43 to 1.84) for those testing positive for marijuana and negative for alcohol, and 6.39 (95% CI: 5.19 to 7.88) for those testing positive for both alcohol and marijuana.ConclusionsAlcohol and marijuana each play a significant role in fatal crash initiation. When used in combination, alcohol and marijuana appear to have a positive interaction effect on the risk of fatal crash initiation on the additive scale.     

Association of alcohol-related laws with deaths due to motor vehicle and motorcycle crashes in the United States, 1980-1997

The authors compared US motor vehicle and motorcycle mortality rates during periods when each of several alcohol-related laws were in effect with mortality rates during other periods. During the period 1980-1997, there were 792,184 deaths due to motor vehicle crashes and 63,052 deaths due to motorcycle crashes. An estimated 26% and 49% of these fatalities, respectively, were attributable to alcohol use. The incidence of alcohol-related mortality in motor vehicle crashes was lower when laws specifying a blood alcohol concentration of 0.08 g/dl per se (laws stating that it is a criminal offense to drive with a blood alcohol concentration above the state's legal limit) were in effect (adjusted rate ratio (RR) = 0.86, 95% confidence interval (CI): 0.83, 0.88). For motorcycle deaths, the adjusted rate ratio was 0.87 (95% CI: 0.79, 0.95). The incidence of alcohol-related mortality in motor vehicle crashes was also lower during periods when two other types of laws were in effect: zero tolerance laws (adjusted RR = 0.88, 95% CI: 0.86, 0.90) and administrative license revocation laws (adjusted RR = 0.95, 95% CI: 0.93, 0.98). Overall motorcycle mortality was lower when administrative license revocation laws were in effect (adjusted RR = 0.95, 95% CI: 0.92, 0.98).