Number 345 April 2008

Traffic Safety Facts

Traffic Tech – Technology Transfer Series

U.S. Department of Transportation 1200 New Jersey Avenue SE., Washington, DC 20590

National Highway Traffic Safety Administration

Effects of Alcohol on Motorcycle Riding Skills

Preliminary estimates indicate that there were 4,810

motorcyclist deaths in 2006, an increase of 5.1 percent

from 2005. Motorcyclist deaths have increased for the

ninth consecutive year and now represent 11.3 percent

of all traffic fatalities in the United States. Moreover, the

number of motorcyclist fatalities has more than doubled since 1997.

One factor that continues to be associated with motorcyclist fatalities is alcohol. In 2005, a higher percentage

of motorcycle operators had blood alcohol concentrations (BACs) of .08 grams per deciliter or higher than

any other type of motor vehicle driver. Twenty-seven

percent of motorcycle operators were at .08 or higher

versus 22 percent for passenger car operators, and 21

percent for light-truck operators.

Despite the relevance of alcohol to motorcycle safety,

only limited information on the impairing effects of

alcohol on motorcycle operator performance is available.To address this issue, NHTSAsponsoreda research

study to assess the impairing effects of alcohol(up to the

current per se limit of .08 BAC), on rider performance

involving a broad set of basic motorcycle riding skills.

Methods

Twenty-four male participants age 21 to 50 (MEAN=32

years) completed three test days for this experiment.

All participants had a minimum of five years of riding experience (MEAN=14.97 years), drank alcohol at

least once a week, and had no history of medical or

psychological problems (i.e., alcohol dependence) 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 (.00, .02, .05, and .08) and

completed one level per test day.

A motorcycle test course was developed in conjunction

with two certified motorcycle coach instructors from

the Minnesota Motorcycle Safety Center based on standard exercises within the Motorcycle Safety Foundation (MSF) training program, including the MSF Basic

Rider Course and the Experienced Rider Course. The

test 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 resulting set of task scenarios that comprised the test course included weaving (slalom) around pylons, hazard avoidance, curve

negotiation, and emergency stops. The test motorcycle

was an instrumented 2000 Honda Shadow VT1100

equipped with outriggers and sensor equipment for

data collection.

Data was collected fortwo sober baseline rides and two

tests rides each day at one of four conditions (BACs of

.00, .02 , .05 or .08). 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.

Results

The results showed that performance for several dependent measures of riding performance were impaired at

different BAC levels.
In the offset weave (slalom) task, participants missed

or hit more pylons and had smaller passing distances

around the pylons in the .08 condition compared to the

other alcohol and .00 conditions. In the hazard avoidance task where a warning was provided 1.5 seconds

from the hazard, participants had slower reaction times

in both the .08 g/dL and .05 g/dL conditions compared

to the .00 and .02 conditions.

In the hazard avoidance task where a warning was provided 2.5 sec from the hazard, participants in the .08

and .05 conditions passed at a closer to distance to the

obstacle than in the .00 or.02 conditions. For both hazard

tasks,riders turned in the wrong direction more often in

the .08 condition as compared to the other conditions.

In the curve circuit task, although not significant, participants in all alcohol conditions tended to have faster

maximum speeds and increased variability in speed in

the circuit compared to the .00 condition. Participants

in the .08 condition were also more likely to cross outside the curve circuit boundaries than participants in

other conditions.

Despite these differences, the effect sizes (Eta2) calculated for the significant main effect of alcohol on all the

tasks was modest, ranging from 2 to 8 percent.

Participants reported requiring more effort to ride

and complete the tasks in the .08 condition when compared to the .00 condition. Their levels of subjective

intoxication also increased significantly with increasing BAC levels. Participants reported their perceived

level of performance impairment was higher for the

.05 and .08 conditions compared to the .00 and .02

conditions. Participants in the .05 and .08 conditions

also reported they would be less willing to ride a

motorcycle for any reason.

Discussion

This study demonstrates changes were obtained in

operator control and rider perception that are indicative of alcohol impairment. Most of the impairing

effects on riding performance and perception were

evident at the per se alcohol limit of .08 BAC; however,

some of these same impairing effects were evident in

the lower .05 condition.

Given that this study used experienced riders performing highly practiced tasks on a closed course at low to

moderate BAC levels, it is not surprising that the effects

of alcohol on motorcycle control and rider perception

were modest. The effects of alcohol were most pronounced when task demand was higher (offset weave),

time pressure was higher (hazard avoidance for near

obstacles), and tolerances were constrained (circuit

track). Larger impairments may be expected with less

experience riders, on less familiar roads, with more

complex and novel tasks at higher alcohol doses.

In summary, study findings indicate that even for experienced riders alcohol has a deleterious effect on performance involving basic riding skills on a closed course.

Other on-road efforts that can assess rider crash risk

and impairment at different BAC levels are needed to

increase our understanding of the contribution of alcohol to motorcycle crashes.

How to Order: To order Effects of Alcohol on Motorcycle 

Riding Skills, write to the Office of Behavioral Safety

Research, NHTSA, NTI-130, 1200 New Jersey Avenue

SE., Washington, DC 20590, fax 202-366-7096, or download from www.nhtsa.dot.gov. Marvin Levy, Ph.D., was

the contract manager.

TRAFFIC TECH is a publication to disseminate information about

traffic safety programs, including evaluations, innovative programs, and new publications. Feel free to copy it as you wish. If you

would like to receive a copy, contact Patricia Ellison-Potter, Ph.D.,

Editor, fax 202-366-7096, e-mail: traffic.tech@dot.gov.

1200 New Jersey Avenue SE., NTI-130

Washington, DC 20590

Effect of BAC condition on reaction time in the neardistance (1.5 seconds) hazard avoidance task.

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Time (s)

.00 .02 .05 .08

BAC Condition

0.45 0.45

0.5 0.5