Chapter 9 - Looking to the Future
Introduction
As traffic calming develops, we must look at the emerging trends. A strong resource for this is the ITE’s State of the Practice (SOP) book, developed by Reid Ewing in September of 1999. The following information presented in this section was found from the ITE’s web site, as well as in their book.
Traffic Calming Trends
Like any other field, traffic calming evolves by learning from mistakes and through trial-and-error. Starting with its routes, traffic calming was limited in the different methods employed. However, streets, traffic problems, and neighborhood preferences are not all alike, so traffic calming treatments must vary accordingly. The traffic calming programs begin with a few favorite measures, followed by experimentation with different measures. For example, West Palm Beach, Florida, started with street closures, then incorporated traffic circles, humps, chokers, and raised intersections into its toolbox. This is how traffic calming evolves into a broad field with multiple options.
Diversification
Diversification uses multiple measures along a single street, and can also be used at a single slow point. An example of this is Hart Road, in Beaverton, Oregon, which uses a speed table and choker in combination (see Figure 9.1).
Figure 9.1: Speed table and choker in combination
When curb extensions were added to the existing speed tables, it had a modest affect on the 85th percentile speeds, but clipped the very highest speeds.
Speed Control
Another trend of traffic calming is shifting from volume to speed control. Early traffic calming programs relied almost entirely on closures, diverters, and other volume control measures. Now nearly all programs are heavily employing humps, circles, and other speed control measures. Volume control tends to divert traffic problems from one local street to another, and communities are showing an interest in avoiding this types of traffic control. A solution is to calm higher order streets without diverting the traffic to other roads. Engineers are trying to do this where speed controls are acceptable, but where volume control would not be.
Slow Point Spacing
The next new trend in traffic calming is the correction from wrong to right spacing of slow points. Early traffic calming programs spaced slow points so far apart that speed mid-block speed control did not work. These distances were as large as 1,000 feet. Motorists would accelerate between slow points to higher speeds than safe as if to make up lost time.
With closer spacing there is less tendency to accelerate because any acceleration is followed immediately by deceleration. A field study including streets in Denmark, Australia, and Britain shows that in order to maintain midpoint speeds of 25mph, slow points must be no more than 300-400 ft apart (see Figure 9.2).
Figure 9.2: Speed vs spacing of slow points from R. Ewing, Best Development Practices, American Planning Association (in cooperation with the Urban Land Institute), Chicago, 1996, P.64.
Interestingly, a vehicle that accelerates between slow points and decelerates at the slow point travels the same distance in the same amount of time as a vehicle maintaining a constant speed along the entire road. This is most easily understood by looking at Figure 9.3. The area under each curve is the same, meaning the vehicles require the same travel time to drive the same distance. Vehicle “A” undergoes brisk acceleration and deceleration, where as vehicle “B” maintains a constant speed. The slow speeds of Vehicle “A” occur at the slow points, and the high speeds occur in-between.
Figure 9.3: Vehicle speeds correlating to spacing of slow points:
Properly spaced slow points will result with speeds like Vehicle B.
Improperly spaced slow points will result with speeds like vehicle A.
Traffic Calming Placed in New Developments
A final trend in traffic calming is moving from costly retrofitting to doing it right from the beginning. Because we spend so much time and money on retrofits, many communities are becoming sensitive to the need to incorporate traffic calming into new developments. New developments have adopted standards with varying features, such as narrowing streets, designs to discourage cut-through traffic, adding roundabouts, and requiring slow points at given intervals.
Three E's of Traffic Calming
In order to ensure the success of any traffic calming measure, the three E's of traffic calming must be employed. The three E's are1:
- Education: the community is informed of the tools and information necessary to make informed decisions regarding traffic concerns. In order to ensure the success of a project, the community must be aware of the changes that will be made, and the reasons for these changes.
- Engineering: The traffic calming measures are implemented based on engineering principals and community input. A strong partnership between the engineers and the community will help ensure the long term success of the changes.
- Enforcement: Community-identified strategies will be supported by police enforcement. The new measures can also attain their goals through increased community awareness.
Technology in Traffic Calming
Some programs are beginning to incorporate “Intelligent Transportation Systems” (ITS) into their traffic calming solutions. Still in its experimental phase, Intelligent Speed Adaptation, or ISA, is being used across seas in countries such as Denmark and Sweden. ISA incorporates various aspects of technology to slow down cars. Some different methods used are2:
Speed Information
Speed Information consists of a box with a green and red diode which the driver can see inside the car while driving. The speed is displayed on the screen which the driver can easily view. If the driver exceeds the speed limit, the red diode eliminates, and an automated voice announces the speed limit. Otherwise, the green diode remains illuminated.
Active Gas Pedal
When a driver attempts to move above the local speed limit, counter pressure will be applied to the gas pedal rending it physically impossible for the vehicle to accelerate above the speed limit. If necessary, the system can be disconnected by pressing down with greater pressure on the pedal.
Transmitters at the Roadside
Transmitters along the roadside can send local speed limits to drivers passing through areas where the speed limit is different than the “normal” speed, such as urban areas where the speed limit drops from 45mph to 25mph.
GPS and Digital Maps
Vehicles containing a global positioning system (GPS) receiver can receive local digital maps with speed limits indicated for each road.
References
- Neighborhood Traffic Management Program: Preserving Sacramento’s Neighborhoods. www.pw.sacto.org/traffic/ntmp.html.
- INFANTI, Different Technologies. www.infati.dk/uk/isadiv.htm
This chapter originally written by Dan Ferster, 2002, as part of a senior project.
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