By the letter of the law, it's illegal to drive faster than the posted speed limit, but many drivers push it by going a few miles per hour over the limit or even much faster.

Drive reader Nancy was recently on a Twin Cities freeway and said she was going about 5% over the posted speed limit. Yet most drivers, she said, were going even faster.

"I find that I no longer really know what speed I should drive," Nancy wrote in an e-mail. "To me the question is what should a safety-conscious driver do to make the roads the safest? Drive exactly the speed limit, slightly over or slightly under?"

The Drive took her questions to Pete Hosmer, driving instructor at A+ Driving School in White Bear Lake.

"We teach that the safest speed on the road is the posted speed limit," Hosmer said. "On local freeways, that is usually 60 mph."

But it's also important to note which lane a driver is in, he said. On roads, highways and freeways with more than one travel lane going in the same direction, the right lane is the "through lane" and should be used most of the time.

The left lane, Hosmer said, is a passing lane — not a fast lane as the speed limit is still in effect — and should only be used when passing a slower car or when making a turn. Of course, there are times when drivers need to use the left lane, such as during rush hour when congestion necessitates its use.

"I do realize that people drive faster than the speed limit all the time, but based on drivers reaction times, following distances and speeds, driving the speed limit and staying in the appropriate lane is key," Hosmer said.

Several states prohibit drivers from camping in the left lane, and say the left lane is to be used only for passing. In Honolulu, the city prohibits motorists from driving more than 5 mph under the limit while in the left lane.

In Minnesota, a "slowpoke law" requires slower drivers to move right to allow other vehicles to pass. But the law does not give leadfoot drivers license to speed in the left lane.

"The speed limit is the speed limit, and drivers should not exceed that," said Lt. Gordon Shank with the Minnesota State Patrol. "If you are a slower driver and there are two lanes, keep to the right to allow traffic moving faster to pass."

Traffic moves most efficiently when drivers are moving at roughly the same speed. Driving too fast or too slow is called a speed variance, and that can contribute to crashes and make the roadway less safe, the Minnesota Department of Transportation said.

So bottom line, Nancy, obey the speed limit and stay to the right to let faster-moving traffic pass by. You'll probably be safest that way.