Chapter 3 - Design Controls
3.3. Design Speed
3.3.1. General Considerations
Design speed is defined by FHWA as a selected speed used to determine the various geometric design features of a roadway (MUTCD, 2000). Design speed should be consistent with the speeds at which 85 percent of drivers are traveling (referred to as the 85th percentile) and likely to expect on the facility.
A design speed that is as high as practical that will provide safety, mobility, and efficiency within the constraints of environmental quality, economics, aesthetics, and other social or political effects should be selected.
On county roads or city streets, GDOT recommends coordination with the local jurisdictional authority on the selection of the posted speed limit and the design speed.
3.3.2. Intersections Approaching a Stopped Condition
To improve the angle of intersection between a local street and major road, a designer may use a lower design speed on the local street for curves approaching an intersection if it is not anticipated that the T intersection will become a full intersection
The design speed of the last curve prior to the intersection may be 10 mph less than the design speed of the local street.
3.3.3. Freeway Exit and Entrance Ramps
Typical freeway exit and entrance ramps may have varying design speeds which are based on the operating speed of the vehicle as it decelerates or accelerates on the ramp. A common rule to apply for ramps is that the design speed of the first curve of an exit ramp can be assumed to be 10 mph less than the design speed of the mainline. With each successive curve on the exit ramp, the design speed of the curve can be reduced based on computed vehicle deceleration. The reverse condition applies to the design speed for all entrance ramps.
The design speed for a direct system to system ramp that connects two freeway facilities shall be no less than 10 mph below the design speed of the exiting facility.
On loop ramps, adequate deceleration length shall be provided prior to the loop part of the ramp. All areas of deceleration shall be separated from the mainline lanes. System to system loop ramps will be evaluated on a case by case basis.
3.3.4. Urban Subdivision Streets
The design speed for urban subdivision streets shall be 25 mph minimum.