The Cobb County Department of Transportation makes a recommendation for posted speed limits for the Radar and Speed Limit Ordinances for review and approval by the Georgia Department of Transportation (GDOT) and the Cobb County Board of Commissioners. Only GDOT approved speed limits may be enforced by Public Safety. The radar and speed limit ordinances are updated approximately every two years. The criteria used by both transportation departments to establish the posted speed limit is based in part upon the characteristics of the roadway and its design speed. The key elements include: road alignment, sight distance, lengths of merges and tapers, road surface characteristics, shoulder condition, grade (i.e., steepness of the road), roadside development, parking practices, driveway spacing, functional classification of the street, and crash experience. Additionally, speed limits are established in part by drivers themselves. In order for a speed limit to be effective, it must be reasonable and the majority of drivers must voluntarily comply. Most drivers tend to regulate the speed of their vehicle relative to road, traffic and weather conditions. National studies have found that the speed at which 85 percent of the motorists travel is reasonable and safe. The determination of the 85th percentile speed is made by conducting a speed survey of vehicles traveling along the roadway in question during normal operating conditions.