Drivers in the St. Louis metro area can cross a state line into Illinois in minutes. However, the rules for suspected drunk driving and sobriety testing shift the moment you leave one bank of the river.
BAC limits for adults, commercial drivers and teens
Missouri and Illinois alike set the legal BAC limit at 0.08% for drivers age 21 and over, while commercial license holders must stay below 0.04%. Under-21 rules diverge sharply: Missouri uses a 0.02% “zero-tolerance” limit, while Illinois treats any detectable alcohol as a violation.
Field sobriety tests: you may refuse, but it could be risky
Officers in either state will likely ask motorists to perform walk-and-turn or eye-tracking tests. Drivers can decline, yet refusal can fuel probable cause for arrest in Illinois and will be admissible evidence in Missouri courtrooms. No separate statutory penalty attaches to refusing these field tests, but the decision often sets up the next fork in the road: chemical testing.
Implied consent and chemical tests
Handing over your keys means you have already “consented” to breath, blood or urine analysis, if lawfully arrested. A Missouri driver who declines a breath, blood or urine test faces an automatic 12-month loss of driving privileges, a penalty the state labels a “Chemical Revocation.” In Illinois, a first refusal yields a one-year suspension and a second within five years bumps that to three years.
Look-back periods
Missouri “forgets” a prior DWI after five years for sentencing purposes. Illinois, on the other hand, remembers much longer: a 20-year window applies when courts calculate penalties for a second DUI.
Interstate driver’s license compact
Because both states belong to the Interstate Driver’s License Compact, a conviction in one will follow you across the river. Missouri will honor an Illinois suspension, and vice versa, so you cannot outrun a DUI by changing license plates.
Navigating two legal systems in one daily commute requires knowing which rules apply the moment you cross the state line. Staying informed, and sober, keeps you on the right side of both jurisdictions.