Impaired Driving and BAC Laws — DMV Permit Practice Questions

Interstate speed limits vary by state and by specific road section. Most US states post Interstate limits in the 65-75 mph range. Some western states allow 80 mph on rural Interstates (Texas, Utah, Idaho, Montana, Wyoming, Nevada, South Dakota). Texas has a 85 mph posted limit on some rural toll roads — the highest in the country. Within urban areas and through cities, Interstate limits typically drop to 55-65 mph. No US state has unlimited Interstate speeds; the federal government previously imposed national 55 mph and 65 mph caps, both later repealed, leaving the matter to individual states. Always check the specific posted limit for the section you are driving — limits change without obvious markers.

When no speed limit is posted, every state has 'default' or 'absolute' limits based on the road type. Typical defaults: 25 mph in business and residential districts (some states 30 or 35); 55-65 mph on rural highways; 65-70 mph on Interstates. School zones during posted hours are typically 15-25 mph. Alleys are typically 15 mph. These defaults are part of state vehicle code and apply automatically when signs are not posted or are obscured. The 'basic speed law' applies on top of these limits: even at the legal limit, you can be ticketed for driving too fast for conditions like rain, fog, or heavy traffic.

Yellow advisory speeds (typically square or diamond yellow signs with a speed number) are recommendations for navigating an upcoming curve, ramp, or hazard. They are not legal limits in the same way black-on-white speed limit signs are — but driving above the advisory speed and crashing is strong evidence of unsafe driving. For passenger cars in good conditions, the advisory is calibrated to be a safe maximum; for commercial vehicles, especially tankers and high-profile vehicles, the safe speed is lower. In wet, icy, or otherwise compromised conditions, the safe speed is lower for any vehicle. The basic speed law requires you to drive at a speed safe for conditions, which may be below the advisory.

At a T-intersection, the road that does not terminate is the 'through' road, and traffic on it has the right of way. The driver on the road that ends at the intersection must yield to all traffic on the through road before turning. This rule applies whether the terminating road has a stop sign (which most do) or not. A yield sign on the terminating road requires only that you slow, look, and proceed when safe; a stop sign requires a complete stop first. Even without any sign on the terminating road, the through-road traffic still has priority in most state laws — though this is rare in practice because such intersections almost always have signs.

Entering an Interstate is the responsibility of the merging driver, not the Interstate traffic. Use the on-ramp's acceleration lane (also called the 'merge lane') to bring your speed up to match prevailing Interstate traffic — usually 60-70 mph. Signal early, check mirrors and blind spots, find a safe gap, and merge smoothly. Stopping at the end of an on-ramp is dangerous: you must then accelerate from zero into fast-moving traffic, which is much harder and more likely to cause a collision. Interstate traffic has the right of way and is not required to slow for you. If the merge lane is too short to reach Interstate speed, drive as fast as safely possible and wait for a clear gap.

Most states allow a right turn on red after coming to a complete stop and verifying it is safe — but several specific conditions apply: come to a complete stop (rolling stops are illegal and frequently ticketed); yield to all approaching traffic from the left; yield to pedestrians in the crosswalk; check for cyclists in the bike lane to your right; obey any 'No Turn on Red' sign that overrides the default. Some states (notably New York City) prohibit right-on-red as a general default with exceptions only where signed. Even where allowed, right-on-red is optional — if you are uncertain or visibility is poor, waiting for the green is always safe.

Most US states permit a left turn on red only when both the street you are leaving and the street you are entering are one-way streets where the destination street's traffic moves to the right (in your direction). After a complete stop and yielding to all traffic and pedestrians, the left turn is allowed where state law permits and no sign prohibits it. A few states do not allow left-on-red at all; others allow it from a two-way street onto a one-way street. Check your specific state's rule. Even where permitted, left-on-red is unusual and not all drivers expect it; proceed with extra care. Posted signs override the default.

Left turn procedure: position your vehicle in the leftmost lane that permits left turns (which may be a dedicated turn lane or the leftmost through lane); signal at least 100 feet in advance (more on highways); come to a stop if required or yield to oncoming traffic if turning across opposite-direction traffic on a green light without arrow; complete the turn into the leftmost lane of the new road's same direction. Crossing over multiple lanes during the turn is illegal and dangerous — complete the turn cleanly into one lane, then change lanes after the turn if needed. Green-arrow signals give protected left turns where oncoming traffic is stopped; round green lights still require yielding to oncoming traffic.

The 'keep right except to pass' rule is the standard for multi-lane highway driving in every state. The right lane is for normal driving; left lanes are for passing. After completing a pass, return to the right or middle lane. Some states actively enforce left-lane camping with fines. The reason: lane discipline reduces lane-changing, and most multi-vehicle highway crashes involve lane changes. Slow vehicles in the left lane force faster vehicles to pass on the right, which is more dangerous than passing on the left. Always pass on the left when possible. On three-lane highways, the middle lane is for through traffic at the prevailing speed.

HOV lanes (also called 'carpool lanes' or 'diamond lanes' from the painted diamond markings) are restricted to vehicles with a specified minimum number of occupants — typically 2+ or 3+ depending on the road and time of day. The restriction usually applies during rush hours; outside posted hours the lane may be open to all traffic. HOV lanes are intended to encourage carpooling and reduce overall congestion. Some HOV lanes accept hybrid and electric vehicles regardless of occupancy under various clean-vehicle programs. Penalties for HOV violations are significant, often $300-$500 in many states. Diamond markings on the pavement and overhead signs identify HOV restrictions.