U.S. Traffic Law Library
The traffic laws that appear on the DMV permit knowledge exam are remarkably consistent across states, but the specific numbers can differ. Right of way conventions, parking distances, BAC limits, school zone speeds, and move over rules form the backbone of nearly every state's exam. Use these topic guides to understand the underlying rule, then drill the state-specific numbers in your state's practice test.
Right of Way
Right of way rules determine who proceeds first when paths cross. They apply at four way stops, T intersections, roundabouts, on ramps, and crosswalks. Right of way is something you give, not something you demand.
Speed Limits
Speed limits are the maximum legal speed under ideal conditions. The basic speed law requires drivers to slow below posted limits when conditions such as rain, fog, traffic, or visibility require. Posted limits do not give permission to drive recklessly.
Parking Rules
Parking rules govern how close you may park to fire hydrants, intersections, crosswalks, driveways, and railroad crossings. They also restrict parking in front of accessibility ramps and on the wrong side of the street.
Alcohol and Driving
Driving under the influence is the leading preventable cause of traffic fatalities. Every state sets a per se BAC limit, with reduced limits for commercial drivers and zero or near zero tolerance for drivers under 21. Implied consent laws mean refusing a chemical test triggers automatic license suspension.
School Zones and Buses
School zones impose strict reduced speed limits when children are present or during posted hours. Stopping for school buses with extended stop arms is required on undivided roadways in every state, with strong penalties for violations.
Intersections
More than half of all crashes happen at intersections. Knowing how to position your vehicle, when to signal, when to yield, and how to handle controlled and uncontrolled intersections is central to passing the permit exam.
Pedestrians and Bicycles
Pedestrians always have right of way in marked or unmarked crosswalks. Cyclists are entitled to use the full lane in many situations, and most states require drivers to give at least three feet of clearance when passing.
Emergency and Move Over
When emergency vehicles approach with sirens or flashing lights, you must pull to the right and stop. Move over laws require changing lanes or slowing significantly when passing stopped emergency, tow, or maintenance vehicles on the shoulder.
Railroad Crossings
Railroad crossings demand extra caution because trains cannot stop quickly and crashes are almost always fatal. You must stop between 15 and 50 feet from the nearest rail when warned, and never cross until you are certain you can clear the tracks.
Why traffic laws are mostly federal in origin
Although traffic enforcement is a state matter, most U.S. traffic law derives from the federal Uniform Vehicle Code, a model law adopted with local modifications by every state. That is why a four way stop in Florida works the same as a four way stop in Oregon, and why a yield sign means yield in every state. The state DMVs adopt the underlying rules and then attach state-specific numbers, fine schedules, and license restrictions on top.
Where states diverge
The largest state-by-state divergences are in BAC enforcement, hands-free phone law, school zone speed thresholds, primary versus secondary seat belt enforcement, and right turn on red exceptions. PermitPrep flags these in every state's state hub page. If you study with PermitPrep but plan to test in a different state than the one you read about, double check the state-specific numbers in your destination state.