A method to avert an unlawfully intoxicated driver from operating a vehicle is provided. The method utilizes a passive ethanol vapor sensor to measure ethanol vapor concentration in air from a vehicle cabin and imposes a safety response when the passive ethanol vapor sensor detects that a sample of vehicle cabin air indicates that a vehicle occupant exceeds the legal blood alcohol concentration (BAC) for a motor vehicle driver. The ethanol vapor sensor may have a passive measurement mode and an active breathalyzer mode. Ethanol vapor in a vehicle cabin may be passively measured and if a predetermined ethanol level is measured, a countermeasure is invoked to improve safety. An active breathalyzer may be used as a countermeasure. The active breathalyzer can be imposed for a number of vehicle trips or for a predetermined time period.