Impairment Begins at .01
The National Agenda for Motorcycle Safety, a set of recommendations developed by MSF, AMA, NHTSA, and independent motorcycling advocates, stated in 2000: “It is unknown at precisely what [blood alcohol] levels motorcycle-specific judgment and skills are impaired.” In its
2006 Motorcycle Safety Program Plan (1.7MB PDF), NHTSA reported that it is conducting an experiment to measure the effects of impairment on motorcyclists at different BACs. My small contribution to answering this question seems to show that for some riders, and to some extent, impairment begins at .01 BAC.
It has been suggested that drunk riders are more likely to be involved in single-vehicle crashes than are sober riders. That would make sense because multiple-vehicle crashes are usually caused by other motorists while single-vehicle crashes are usually the rider’s fault. To see if this hunch is supported by facts, I tallied rider BAC and crash type (single-vehicle or multiple-vehicle) for 26,000 fatal motorcycle crashes in the US from 1994 to 2006. The result is seen in the attached chart.
At each measured BAC level, the percentage of riders killed in single- vs. multiple-vehicle crashes is shown. At .00 BAC, one-third were in single-vehicle crashes and two-thirds were in crashes with two or more vehicles. At .15 BAC and higher, two-thirds were single-vehicle and one-third were multiple-vehicle. From .01 to .14, higher BAC is associated with a higher percentage of riders in single-vehicle crashes.
If we can assume that rider impairment is more likely to cause a single-vehicle motorcycle crash than a multiple-vehicle crash, then it clearly begins at .01 and increases steadily with BAC. But why would that be?
Physical impairment doesn’t start that low. Vision, balance, and motor skills aren’t affected until a higher BAC.
Judgment, however is a different story.
You’ve probably been out drinking with a group where some guy boorishly hits on the cocktail waitress after the first round, guaranteeing crappy service for the rest of the night. Yet other drinkers maintain decorum round after round, even after their motor skills are woefully deteriorated. As I see it, this individual variation in judgment under the influence of alcohol accounts for the apparent low onset of impairment. In the article
Rolling Stoned,
Motorcycle Cruiser magazine reports on an experiment that demonstrated the effects of impaired judgment on drinking riders. Some subjects exhibited improved skills at low BAC but their judgment deteriorated early.
The role of impaired judgment in spite of fully functional physical skills presents a problem for programs that attempt to reduce alcohol-related motorcycle crashes. It’s easy for someone to understand why he shouldn’t ride if he can’t see straight or keep the motorcycle upright. It’s more difficult when he’s fully capable of controlling the bike but lacks the good sense to ride safely. I don’t think lower BAC limits are the answer, because the results seen here don’t reveal a characteristic of
all riders, just
some. In fact, I would guess that most motorcyclists can ride safely at any level of intoxication that doesn’t significantly affect their perception or motor skills. Rather, I think the message in the data is that judgment can be impaired long before a rider notices his physical skills deteriorating and that people need to be aware of their limitations when drinking.
If you’re the kind of person whose inhibitions loosen easily with alcohol, consider the vast difference in the consequences of a social
faux pas and of overestimating your riding abilities.