What the Dope!

Contact name: 
Mark B. Johnson
Centre Director
Organization: Pacific Institute for Research and Evaluation, Calverton Center.
+1 301-755-2700
Address: 11720 Beltsville Drive, Suite 900, Calverton, MD 20705
+1 301-755-2799



Given that most effective alcohol harm-reduction laws specify the blood alcohol concentration (BAC) that constitutes illegal behaviour (e.g., the .08% breath alcohol concentration legal limit), interventions that allow drinkers to accurately estimate their BACs, and thus better assess their risk, have potential importance to long-term driving-under-the-influence prevention efforts.


This study describes a field experiment designed to test the impact on drinking of providing “Know Your Limit” (KYL) BAC estimation cards to individuals in a natural drinking environment.


1,215 U.S. residents were randomly sampled as they entered Mexico for a night of drinking, interviewed them, and randomly assigned them to one of six experimental conditions. Participants were re-interviewed and breath-tested when they returned to the United States. The experimental conditions included providing generic warnings about drinking and driving, giving out gender-specific BAC calculator cards (KYL cards), and providing incentives to moderate their drinking.


Cueing participants about the risks of drunk driving resulted in significantly lower BACs (relative to control) for participants who indicated that they would drive home. Providing KYL matrixes did not reduce BACs, and, in fact, some evidence suggests that KYL cards undermined the effect of the warning.


KYL information does not appear to be an effective tool for reducing drinking and driving. Implications for prevention and future research are discussed.

Intervention details

Type of intervention
Brief Intervention.
Intervention setting
Target population

U.S. residents who cross the Ysidro border (crossing between San Diego County, CA, and Tijuana, Mexico) to drink

Substances adressed
Intervention activities
Providing information
Enforcement: breath tests
Participant first answered a survey form and received a hospital-style identification bracelet to identify them subsequently. Afterwards breath samples were collected. Returning participants were given an oral interview, and their responses were recorded by the interviewer. After completing the interview, participants were asked again to provide an exit BAC sample.
Theory/evidence behind the intervention

Prevention programs targeting young adults have used numerous environmental approaches, from media campaigns to law enforcement and combinations of the two (Clapp et al., 2006). Although broad, mass media approaches to the reduction of drinking often provide general information on alcohol risks, individuals may lack a method for reacting appropriately to the message content. This tends to be true of messages relating to laws setting an illegal BAC level, perhaps because individuals have no direct method to assess their own BAC levels. In response to this problem, some countermeasure programs attempt to make risks salient by providing drinkers with the tools to obtain more accurate estimates of their BAC levels (Royal, 2000). State driver’s education classes and “Alcohol 101”–style programs offered to college students also teach BAC estimation by instructing young people how to associate alcohol symptoms to approximate BACs (e.g., Larsen and Kozar, 2005; Sharmer, 2001; www.centurycouncil.org/see-our-work/evaluation/alcohol101plus).

Number of people needed
Individual approach
Time required to run
Whole drinking night (start before crossing the border and ends between 1am and 5am when young people come back)
Other resource requirements

Breath Analyzer

Evaluation details

Evaluation type (e.g. process, outcome, cost-effectiveness)
Outcome evaluation
Activities evaluated

1) All participants received information cards about the drunk-driving enforcement operations being conducted in the San Diego area.
2) Motivation condition. These participants received the warning about police activity and were informed that they receive a incentive when they return with a BAC of less than .05% Participants in three of the treatment conditions were given a wallet-size card typically called “Know Your Limit” (KYL). Information about the relationship of gender, weight, and the number of drinks in a fixed period to the estimated BAC level of the drinker. One group only received the card, the other group received a card as well as explicit instructions on how to use the card, and was instructed to use the card at least twice during the evening to estimate his or her BAC. In one additional group participants received the KYL as well as the generic warning and were also offered the incentive to limit their drinking and return with a BAC of less than .05%.

Type of evaluator (e.g. external consultant, internal evaluator)
Internal evaluator
Evaluation results (Outcome evaluation)

In general, providing drinkers with KYL information—from which they could estimate their BACs—did not play a role in reducing drinking behaviour. For non-drivers, pairing KYL information with a general warning about drinking and driving risks clearly failed to reduce BrACs more than did the general warning alone, and marginal trends suggest that the KYL information might have increased consumption. Simply warning individuals about the risks of drinking and driving was sufficient to reduce BrACs of drivers. This warning, however, also appeared to increase the BrACs of non-drivers.

Evaluation references

Johnson, M. B., & Clapp, J. D. Impact of providing drinkers with 'know your limit' information on drinking and driving: A field experiment. J. Stud. Alcohol Drugs, 72, 79-85, 2011