Preliminary breath test laws allow the police to administer a breath test for blood alcohol concentration without first arresting the driver. The test can be given on the highway without the help of medical personnel. Drivers who pass the test are free to go without further delay and without a record of arrest. The present research used four highway fatality rates as measures of drunk driving. It also used two types of analysis to test the effects of a preliminary breath test law with 4 independent variable models and 12 dummy variable models. The results showed that the passage of a law on breath testing has a significant deterrent effect on drunk driving. Simulations using these results also suggested that if all states had a preliminary breath test law, highway fatalities could be reduced by about 2,000 per year. Tables, appended list of data sources, and 13 references.