What equipment is commonly used to measure a driver's BAC?

The breathalyzer is the equipment commonly used to measure a driver's blood alcohol concentration (BAC). This device estimates the level of alcohol in a person's breath, which correlates to the BAC in their bloodstream. When a driver is suspected of driving under the influence, law enforcement officials often administer a breath test using a breathalyzer during traffic stops. The accuracy and efficiency of breathalyzers make them a standard tool in DUI investigations, as they provide immediate results that aid officers in determining whether a driver is impaired.

In contrast, the other options listed serve different purposes. A speedometer measures the speed of the vehicle, a blinking light tool assists in signaling or directing traffic, and a tachometer gauges the engine's revolutions per minute (RPM). None of these instruments relate to assessing alcohol consumption, which is why they are not suitable for measuring BAC.