Roads are designed to achieve a desired travel speed of vehicles. The challenge of CSS is to plan and design the road layout and network so that the actual speed of vehicles is appropriate to the function of a road and its context. This is sometimes called speed management. Design assumptions about speeds are critical to CSS projects.
Design Speed "All geometric design elements of the highway are affected by design speed. Some roadway design elements are related directly to and vary appreciably with design speed such as horizontal curvature, super-elevation, sight distance, and gradient. The selection of a particular design speed is influenced by the following factors: functional classification of the highway; character of the terrain; density and character of adjacent land uses; traffic volumes expected to use the highway; economic and environmental considerations."
Flexibility in Highway Design
Selecting a Design Speed
Best practices call for designers to select a design speed that is high enough so that most drivers will travel at or lower than the design speed, but low enough so that the physical effects of the design will be manageable and acceptable.
NCHRP Report 480: A Guide to Best Practices for Achieving Context Sensitive Solutions
Design Speed, Operating Speed, and Posted Speed Relationships "Concerns arise at locations where the posted speed limit based on an 85th percentile speed exceeds the roadway's inferred design speed. This inconsistency is a result of the fact that criteria used in highway design incorporate a significant factor of safety--that is, roadways are designed for near worst-case conditions."
Article / Paper / Report
An Improved Traffic Environment, A Catalogue of Ideas
"The catalogue contains a brief description on the development of the traffic calming concept, the planning process, a short account of what can be achieved by reducing car traffic volume and lowering car speed, introduction of the concept of traffic management by design, the elements of urban space and speed levels - streets and roads, squares, points - from both large and mall towns, cheap and expensive solutions and both wholly and partly implemented schemes. Each example includes a description in text and pictures of the conversion and the results achieved."
-- Lene Herrstedt, Kenneth Kjemtrup, et al., Danish Road Directorate
Synthesis of Safety Research Related to Speed and Speed Management
"This report presents a synthesis of research findings on the safety effects of speed, speed limits, enforcement, and engineering measures to manage speed. The report updates a similar synthesis published in 1982. A great deal of speed related research has been carried out since that time. This synthesis highlights the results of significant safety research related to speed completed since the last update. Some of the earlier benchmark studies are incorporated where appropriate."
National Highway Traffic Safety Administration
Federal Highway Administration
Merritt Parkway Gateway Project Greenwich, CT
Since its opening in 1940, the Merritt Parkway has been recognized not only as an essential component of Connecticut's transportation system but as an asset with unique design features and scenic character. During the 1990s, ConnDOT sought to improve the Parkway's safety and operational efficiency while preserving the road's unique characteristics. This case study illustrates this project and the importance of framework development, being flexible in the use of design criteria, and addressing safety problems with specific actions.
Towson Roundabout Towson, MD
Towson, Maryland is a suburb of Baltimore, Maryland. Near the central part of the Towson business district, four major arterials converge at a single location. The awkward, multi-leg signalized intersection caused congestion and safety problems. In addition, the business community and City of Towson sought improvements to the economic viability of the downtown and believed that economic improvements were directly related to traffic improvements. A number of alternatives were developed, and eventually, a signalized roundabout, relatively new to the U.S. at the time, emerged as the preferred solution. The roundabout and streetscape project are considered a major success and are a source of local pride in the town of Towson.
"Despite the substantial social and technological changes that have occurred since the original speed synthesis was published , vehicle speed remains an important public policy, engineering, and traffic safety issue. Speed is cited as the related factor in 30 percent of fatal crashes and 12 percent of all crashes ... Excessive vehicle speed reduces a driver's ability to negotiate curves or maneuver around obstacles in the roadway, extends the distance necessary for a vehicle to stop, and increases the distance a vehicle travels while the driver reacts to a hazard."
"...Straighter wider roads encourage greater speed. Accidents that do happen are therefore more severe, resulting in more injuries or a greater likelihood of death. The most successful measures in reducing the road toll are those which 'force a level of car use in a direction away from the available limits of the car and the driver'. In other words, force drivers to drive at speeds and in a manner which are below the safety limits of the car and the road."
"An alternative design approach that attempts to achieve greater consistency among design speeds, actual driving speeds, and posted speed limits is under development in the United States. The idea is to design roads to ensure that driver operating speeds are consistent with a target operating speed."
"Operating speed is the speed at which driver are observed operating their vehicles during free-flow conditions. The 85th percentile of the distribution of observed speeds is the most frequently used measure of the operating speed associated with a particular location or geometric feature."