Effects of Alcohol on Motorcycle Riding Skills

The following document has been taken from a resource link attributed to the US National Highway Traffic Safety Administration (NHTSA).

Abstract

Alcohol is known to disrupt the effect of neurotransmitters and impair various psychomotor skills. Indeed, alcohol intoxication is a significant risk factor for fatal traffic crashes, especially when riding a motorcycle. At present, there is sparse research on the impairing effects of alcohol on skills involved in motorcycle control. This study was designed to measure the effect of alcohol (up to a blood alcohol concentration of .08 grams per deciliter) on a broad set of basic riding skills. These riding skills were assessed on a test track with task scenarios based on the Motorcycle Safety Foundation’s training program. This study used a balanced incomplete block design to remove confounding artifacts (learning effects) by randomizing four BACs across three test days. Performance was characterized in terms of riding strategy used to cope with the effects of alcohol as a neurological stressor and the amount of resulting impairment with reference to specified performance standards. The analysis controlled for rider gender and age, riding skill, and drinking history. The results showed there were observable changes in motorcycle control and rider behavior in response to alcohol that are indicative of impairment. In general, intoxicated riders demonstrated longer response times and adopted larger tolerances leading to more task performance errors. Riders appeared to protect bike stability at the expense of other task performance and riders tried harder -- where possible -- to fully or partially compensate for the negative effects of alcohol. Most of the alcohol effects were evident at the per se BAC .08 g/dL level, but some of the effects were observed at the lower BAC .05. Given that this
study used experienced riders performing highly practiced tasks with low to moderate levels of alcohol, the effect of alcohol on motorcycle control and rider behavior were modest except when task demand was high (offset weave), time pressure was high (hazard avoidance for near obstacles), and tolerances were constrained (circuit track). The practical significance of the findings was discussed in terms of study constraints.

Report Date

December 2007

Executive Summary

Alcohol is a greater risk factor for fatal crashes involving motorcycles than other types of vehicle operation (NHTSA, 2006). For example, 1 in 4 automobile driver fatalities in the United States were alcohol-related during 2005. In comparison, a higher proportion of motorcycle rider fatalities (1 in 3) were related to alcohol in the same year (see also Subramanian, 2005). Although researchers have hypothesized that motorcycle riding performance could be impaired at levels below established per se limits (Colburn et al., 1993), there has also been limited research to characterize the impairing effect of alcohol on motorcycle control. The purpose of this study was to observe the effects of different levels of blood alcohol concentrations (BAC) on motorcycle riders’ performance on a closed test course. This study set explicit performance standards for a set of skill-based riding tasks to assess rider ability to maintain that performance at different levels of blood alcohol concentrations. The study also evaluated riders’ subjective perceptions of their level of impairment and intoxication.

Methods

Twenty-four male participants age 21 to 50 (M=32 years) completed three test days for this experiment. All participants had a minimum of 5 years of riding experience (M=14.97 years), drank alcohol at least once a week, and had no history of medical or psychological (i.e., alcohol dependence) problems that would preclude them from participating in the study. The study design consisted of a balanced incomplete block design (BIBD) where participants were randomly assigned to one of four possible conditions. Participants in each condition experienced three out of four possible levels of alcohol presentation (placebo, .02 g/dL, .05 g/dL, .08 g/dL) and completed one level per test day. All testing took place from July 1to Aug. 31, 2006, and under dry conditions only. Testing was postponed and rescheduled for another day if rain occurred on a test day. A motorcycle test course was developed in conjunction with two certified motorcycle coach instructors from the Minnesota Motorcycle Safety Center (MMSC) based on standard exercises within the Motorcycle Safety Foundation (MSF) training program, including the MSF Basic Rider Course (BRC) and the Experienced Rider Course (ERC). This course was designed to include specific task scenarios from these training programs that tested performance of riding skills deemed relevant to the safe control of motorcycles. The task scenarios from these programs (see Table 1 in Methods) were modified when necessary to facilitate data collection, but the premise of each task was preserved. The resulting set of task scenarios that comprised the test course included weaves (slalom) around pylons, hazard avoidance, curve negotiation, and emergency stops (see Figure 2 in Methods). The test motorcycle was an instrumented 2000 Honda Shadow VT1100 equipped with outriggers and sensor equipment for data collection.

Data was collected for two baseline rides and two test rides at one of four alcohol conditions (BAC .00, .02, .05 and .08) each day. Data was also collected for a set of subjective measures that evaluated mental workload for the riding tasks and the riders’ perceived levels of intoxication and impairment. The data analysis used baseline riding performance, riding experience (years), and drinking experience (drinks/week) as covariates. BAC condition was the main independent variable for each performance measure. For all dependent measures, two sets of post-hoc tests (Tukey HSD Test) were completed in response to a significant BAC effect in the ANCOVA model:

  • General Alcohol Effect: The BAC .00 condition was compared to the alcohol conditions (BAC .02, .05, .08) to identify the lowest level of alcohol that significantly affected participants (p
  • Equivalent Alcohol Effect: The BAC .08 condition was compared to all other alcohol conditions (.02, .05) to examine the generalization of alcohol effects (p

    Results

    The results showed that performance for several dependent measures of riding performance were impaired at the BAC .08 condition.

  • In the offset weave (slalom) task, participants missed or hit more pylons and had smaller passing distances around the pylons in the BAC .08 condition compared to the other alcohol and placebo conditions.
  • In the hazard avoidance task where a warning was provided when the motorcycle was 1.5 seconds away from the hazard, participants had slower reaction times in both the BAC .08 and .05 conditions compared to the placebo condition.
  • In the hazard avoidance task where a warning was provided when the motorcycle was 2.5 seconds away from the hazard, participants in the BAC .08 and .05 conditions passed at a closer distance to the obstacle than in the placebo or BAC .02 conditions. For both hazard tasks, riders turned in the wrong direction more often in the BAC .08 condition.
  • In the curve circuit task, there was a significant main effect of BAC for maximum speed and speed variability. Although post-hoc tests were not significant, participants in all alcohol conditions tended to have faster maximum speeds and increased variability in speed in the circuit compared to the placebo condition. Participants in the BAC .08 condition were also more likely to cross outside the curve circuit boundaries than participants in other conditions.
  • In the emergency stop task, participants in the BAC .05 condition reached maximum deceleration faster than participants the other alcohol conditions. This difference was significant between the BAC .05 and .08 conditions. Finally, there was a significant change in motorcycle position during the emergency stopping task between the BAC .08 and .02 conditions, where the BAC .08 condition showed more deviation in their stopping path compared to the BAC .02 condition.

    Participants reported requiring more effort to ride and complete the tasks in the BAC .08 condition when compared to the placebo condition. Their levels of subjective intoxication also increased significantly with increasing BACs. Participants reported that their perceived levels of performance impairment was higher for the BAC .05 and .08 conditions compared to the placebo and BAC .02 conditions. Participants in the BAC .05 and .08 conditions also reported they would be less willing to ride a motorcycle for any reason.

    Conclusions

    This study demonstrates some changes in riding behavior in response to alcohol consumption that may be construed as impairment relative to standard performance and the self-assessment of riders. Most of the impairing effects on riding performance were evident at the per se alcohol limit of BAC .08. However, some of these same impairing effects were also evident in the lower BAC .05 condition. Admittedly, the effect sizes (Eta 2 ) calculated for the significant main effect of alcohol may be considered small (a range of 2% to 8% of variance was accounted for by the alcohol effect). Given that this study used experienced riders performing highly practiced tasks on a closed course at low to moderate BACs, the effect of alcohol on motorcycle control and rider behavior was modest except when task demand was high (offset weave), time pressure was high (hazard avoidance for near obstacles), and tolerances were constrained (circuit track). Larger impairments may be expected with less experienced riders, on less familiar roads, with more complex and novel tasks at higher alcohol doses. Although the participants’ self-reports suggest that riders may be aware of the intoxicating and impairing effects of alcohol, this study cannot conclude that corollary self-regulation would be sufficient to mitigate crash risk. Similarly, more research is needed to determine real-world implications of BAC during the riding experience.

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