Roadways: Design Speed

Design Speed

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

Discussion Discussion Designers should select a design speed which is appropriate for the roadway and that speed is used to determine the various geometric features of the design. The selected design speed should realistically represent actual or anticipated operating speeds and conditions on the roadway being designed. For new construction projects, the anticipated posted should will be used for the design speed. A design speed that is set higher than the anticipated posted speed will lead to a roadway that will provide 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. For reconstruction projects, the designer should refer to the as-built plans to determine the existing design speed for the facility. If as-builts do not exist or cannot be found, the designer can use multiple methods to determine the design speed. 1) The designer can 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. 2) 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. 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 design speed for the project will not reach this goal. If the design speed is lowered for the project and there are no geometric modifications considered, the existing geometric elements of the facility will still meet the existing design speed of the facility and 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 design that are anticipated to be posted at 70mph to set the design speed at a minimum of the anticipated posted speed.