The current state of practice for
designing roadways within the United States is to choose a “design speed” for a
roadway and use that speed as an input to determine other roadway factors. AASHTO defines design speed as follows:
_Design speed is a selected speed
used to determine the various geometric features of the roadway. The
assumed design speed should be a logical one with respect to the topography,
anticipated operating speed, the adjacent land use, and the functional
classification of the highway. _
The definition for “target speed”
is the speed that you intend for drivers to go, rather than “design
speed”. For example, a straight roadway
with 12-foot travel lanes and no curbs in an urban setting may technically have
a “design speed” of 45 mph due to its design characteristics, but the desired
“target speed” for that roadway may be 25 mph.
The topic of “design speed”
versus “target speed” typically centers on roadways with speed limits between
25 mph and 45 mph especially where the 85th percentile speed is
higher than the posted speed limit.
Agencies are often asked the question “What can be done to reduce the
speeds to obtain a desired target speed?” and that question is often difficult
to answer due to the nature of design with speed being an input instead of an
output along with knowing what roadway and non-roadway elements actually
influence a driver’s speed choice.
The purpose of this research is to investigate the
question of what roadway / non-roadway elements influence operating speed. This research would focus on roadways with a
posted speed limit between 30 and 40 mph, typically collectors and arterials
within an urban/suburban context. The
research would review various elements of roadways where the 85th
percentile speed is at or near the posted speed (within 5 mph) and those
roadways where the 85th percentile speed is over the posted speed
limit by 10 mph or more. These elements
could include roadway width, shoulder width, presence of curbs, driveway
density, tree density and size, presence of on-street parking, presence of
on-street bicycle facilities, presence of transit stops, roadway curvature
(both horizontal and vertical), signal density, presence of sidewalks, sidewalk
width and setback, building setback, land use, pedestrian and bicyclist
activity, as well as others. The result
of the research should provide the profession with better knowledge of what
elements may affect speed and how to incorporate those elements into better
· Bassan, Shy, “Highway Design Policy Insights for Target Speed: an Israel Perspective”, Traffic Engineering & Control, Volume 57, Issue 4, 2016, pp 155-158.
· “Traffic Safety Advocates Target Speed”, Traffic Safety (Chicago), Volume 04, Issue 5, 2004, p. 2.
· Braaksma, JP, “Target Speeds for Speed Zoning and Traffic Calming in Residential Areas”, 2001 Annual Conference and Exhibition of the Transportation Association of Canada, September 16-19, 2001, Halifax, Nova Scotia: Partnering For Success In Transportation, 2001.
· Jiang, Zhouton, et al, “Speed Harmonization—Design Speed vs. Operating Speed”, Civil Engineering Studies, Illinois Center for Transportation Series, Issue 16-021, 2016, 92p.
· Deller, J, “The influence of road design speed, posted speed limits and lane widths on speed selection: methodology for simulator and observational research study”, Australian Institute of Traffic Planning and Management (AITPM) National Conference, 2015, Brisbane, Queensland, Australia, 2015, 12p.
· Lazda, Z, et al, Smirnovs, “Application of Design Speed for Urban Road and Street Network”, Baltic Journal of Road and Bridge Engineering, Volume 9, Issue 1, 2014, pp 26-30.
· Deller, J, “The influence of road design speed, posted speed limits and lane widths on speed selection: a literature synthesis”, Australasian Transport Research Forum (ATRF), 36th, 2013, Brisbane, Queensland, Australia, 2013, 14p.
· Choi, Jaisung, et al, Effects of Changing Highway Design Speed, Journal of Advanced Transportation, Volume 47, Issue 2, 2013, pp 239-246.
· Porter, Richard J, et al, Geometric Design, Speed, and Safety, Transportation Research Record: Journal of the Transportation Research Board, Issue 2309, 2012, pp 39–47.
· Faghri, A, et al, Design Speed Selection Recommendations, University of Delaware, Newark, Delaware Department of Transportation, 2004, 92p.
· Harwood, Doug, Alternatives to Design Speed for Selection of Roadway Design Criteria, National Cooperative Highway Research Program, Project 15-25, 2008.
· Stamatiadis, Nikiforos, Analysis of Inconsistencies Related to Design Speed, Operating Speed and Speed Limits, Kentucky Transportation Center Research Report, 2004, 64p
· Fitzpatrick, Kay, et al, Design Speed, Operating Speed, and Posted Speed Practices, NCHRP Report, Issue 504, 2003, 101 p.
· Recent and in-process research, including:
o NCHRP Report 737: Design Guidance for High-Speed to Low-Speed Transitions Zones for Rural Highways
o NCHRP Report 839: A Performance-Based Highway Geometric Design Process
o NCHRP Project 15-48 (Report 880): Guidelines for Designing Low and Intermediate Speed Roadway that Serves All Users
o NCHRP Project 17-58: Safety Prediction for Urban and Suburban Arterials. Report available at: https://ceprofs.civil.tamu.edu/dlord/Papers/FinalReportNCHRP17-58withAppendicesJune_2016.pdf
o Ongoing NCHRP Project 17-76: Influences on Operating Speed
· US Department of Transportation Publications
o Speed Management Toolkit, Federal Highway Administration (FHWA)
o Speed Management Program Plan, National Highway Traffic Safety Administration (NHTSA)
o Reducing Speeding-Related Crashes Involving Passenger Cars, National Transportation Safety Board (NTSB)
o Highway Design Handbook for Older Drivers and Pedestrians, FHWA-RD-01-103
· Other Documents for Consideration
o Designing Walkable Urban Thoroughfares: A Context Sensitive Approach, Institute of Transportation Engineers (ITE)
o Project Development and Design Guide, Massachusetts Department of Transportation
o Highway Design Manual, Washington State Department of Transportation
o Speed Zoning Manual and Florida Design Manual, Florida Department of Transportation
o Towards Safe System Infrastructure: A Compendium of Current Knowledge, Austroads Publications Online
o Safe System Approach in Australia and New Zealand