Speed limits may be less permanent than many people think. As Georgia Public Broadcasting reports, just last year the state Department of Transportation raised the speed limit on several major roadways up to 70 miles per hour.
The Federal Highway Administration explains that before local or state transportation agencies set speed limits or recommend changes to them, they complete engineering speed studies. These studies evaluate many factors to determine a safe and appropriate speed limit.
One of the things that engineers take into consideration is the speed that people are already driving on the road, regardless of the posted speed limit. A Georgia Department of Transportation (GDOT) spokesperson admitted that this was a factor in making the decision to raise the speed limit on major roads around Atlanta.
Engineers may measure the 85th percentile speed, which is the velocity that 85% of drivers travel at or below. Government agencies make decisions based on 85th percentile speed, assuming that a majority of drivers choose speeds appropriate to road conditions.
Transportation agencies do not set speed limits solely on the basis of 85th percentile speed. They also consider hazards that may make a reduction in speed prudent. These include hazards that are always present, such as steep hills or curves.
When there are temporary hazards, such as road construction or inclement weather, a variable speed limit of at least 10 miles per hour below the posted limit may go into effect. These are limits that only apply as long as the hazard exists. They supersede any posted limits, and drivers usually find out about them from changeable message sites erected along the side of the road.