Roadways: Design Speed

Design Speed

Primary Guidance High-speed is considered to be a design speed greater than 45 mph and low-speed less than or equal to 45 mph For all public facilities including interstates, design speed should be based on roadway functional classification and context For Interstates Urban or Mountainous shall meet or exceed 50 mph Rural Non-Mountainous shall meet 70 mph New Roadways will be the anticipated posted speed Design speed is NOT automatically set at 5mph or 10 mph over the posted speed

Discussion Select a design speed appropriate for the roadway and use that speed to determine the various design geometric features. The selected design speed should realistically represent actual or anticipated operating speeds and conditions on the roadway being designed. New Construction Projects The anticipated posted speed will be used for the design speed. A design speed set higher than the anticipated posted speed will lead to a roadway that provides more comfort for drivers to travel at a higher speed than may be desired. The higher design speed will also lead to flatter horizontal and vertical curves as well as larger typical sections. These conservative design elements lead to higher roadway construction costs, long-term maintenance costs, and higher than anticipated speeds. Reconstruction Projects Refer to the as-built plans to determine the existing facility design speed. If as-builts do not exist or cannot be found, the designer can use multiple methods to determine the design speed. Layout a preliminary alignment for the existing facility and determine the design speed in which the existing facility meets based on the latest design criteria. A prevailing speed study can be done to determine the prevailing or 85th percentile speed. This speed can then be used for the design speed of the facility. Considerations The design speed should NOT be set lower than the posted or prevailing speed (based on a speed study). If there is a need for the project to lower prevailing speeds on the facility, simply lowering the project design speed will not reach this goal. If the design speed is lowered for the project and there are no geometric modifications considered, the existing facility geometric elements will still meet the existing facility design speed, and the driver will not be compelled to lower driving speeds. Interstate For the highest type interstates in Maryland, the posted speed can be set up to 70mph. Care should be taken on all existing and proposed interstate designs that are anticipated to be posted at 70mph to set the design speed at a minimum of the anticipated posted speed.

See Also

• Practical Design Implementation Guidance