What does a chemical test measure for in drivers suspected of DUI?

The correct answer focuses on the measurement of substances in the driver's system. A chemical test is specifically designed to determine the presence of alcohol or illegal substances in a person's bloodstream. This is critical in DUI cases, as it provides objective evidence regarding the driver's level of impairment at the time of driving.

When law enforcement suspects that a driver is under the influence, they administer a chemical test—such as a breathalyzer or blood test—to quantify the amount of alcohol or drugs present. These tests are scientifically validated methods that can either confirm the suspicions of impaired driving or exonerate the driver if no substances are detected.

The other options do not pertain to the context of DUI testing. For instance, the measurement of seatbelt use, driving speed, distance, or vehicle maintenance compliance does not relate to assessing a driver’s intoxication level. The focus of chemical tests is solely on the driver's physical state as it relates to substance use, which is why the presence of alcohol or illegal substances is the key aspect being measured.