RALEIGH, N.C. — On some stretches of highway, the posted speed limit is 55. On other parts, it is 65. It can be as high as 70 in still other sections of road.

Who sets the speed limit, anyway?

"The important thing about speed limits is that they are reasonable, and the general public respects them, and that we follow through on them," said Kevin Lacy, of the Department of Transportation.

Lacy manages the Traffic Safety Unit, of the DOT's Traffic Engineering Branch. He said local engineers make the recommendations for the speed limits. The state traffic engineer -- whom Lacy works for -- approves or disapproves the speed limits.

"There is no one rule that fits everything," Lacy said. "It would be ludicrous for us to do the same thing on interstates as we do on secondary roads, and vice versa."

Highway 64 in eastern Wake and Franklin counties just passed the test. The DOT increased the speed limit to 70 last month.

Almost every expert you talk to will tell you when you raise the speed limit, the number of crashes, injuries and deaths goes up, sometimes significantly. But, according to the DOT, North Carolina's increases are smaller than other states, and in some cases, the number of incidents actually has gone down.

DOT statistics show that crash rates went down on U.S. 421 in Forsyth County and U.S. 220 in Montgomery and Randolph counties after the speed limit was raised from 55 to 65. But the statewide rates tell a mixed story.

On all North Carolina interstates with a speed-limit increase, crash deaths decreased 28 percent. But, total crashes went up nearly 15 percent.

The number of tickets issued also goes up when the speed limit goes up.

Flat land, wide medians and shoulders, and few road crossings are major factors that support increasing the speed limit. But the decision comes down to a study by DOT safety engineers. They say they are studying some road along someone's drive every day.