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Design speed and operating speed.

Managing the speed of vehicles operating on our streets and highways is a never-ending struggle. Over the past half century, officials have reacted to this pervasive problem in various ways, many of which seem to repeat from time to time in seemingly cyclic fashion.

As transportation officials, it does not take us long to recognize that the motorists' sensitivity to operating speed varies dramatically, depending on which "hat" they are wearing. As residents, they desire and expect low travel speeds by others on streets in their immediate neighborhood. As motorists traveling in other neighborhoods, they desire to travel faster than they expect others to travel in their neighborhoods.

We have learned that frequent and noticeable enforcement will reduce motorists' speed; however, enforcement alone will not have lasting impact on reducing travel speed. After many years of attempting to control speed through arbitrary speed limit signing, studies finally convinced the professional and, to a lesser degree local officials, that motorist speed is not sensitive to posted speed limits. Additionally, average travel speed is generally not impacted by the arbitrary placement of intermittent stop signs. Prolific use of all-way stop control, once in vogue, was eliminated from most communities in the early 1950s. (It appears as if recent political pressure is forcing the reinstallation of some of these devices.)

Studies have led the operational engineer to adopt speed limit policy established by first measuring the free flow speed of the motorist specific to sections of roadway and establishing the posted speed limit based on the speed that all but the upper 15 percent of the drivers are traveling. Further adjustments have been factored into the policy by recognizing frequency of access and pedestrian activity.

Once human discretion was eliminated, posted speed limits became manageable, resulting in regulation consistent and fair to all.

Paralleling the development and evolution of posted speed limits and the enforcement thereof, street and highway designers have been attempting to design facilities that exhibit minimum "design" speed characteristics. The foundation of design speed is horizontal and vertical alignment. Related physical factors, such as lane width, superelevation, and road surface friction characteristics are also involved.

Since, initially, design speed established the physical limits of a particular design based on a motorist's ability to safely negotiate a section of roadway, the design speed established by the designer was consistent with the operating speed comfortable to the average, conscientious driver.

In rural areas, horizontal curves were ball-banked and posted with advisory speed plates by the operations engineer to communicate an approximation of the design speed.

Within the last few years, design engineers have been including other design features into design speed calculations and evaluations. Unlike the initial design speed features, the recently added features involve off-roadway impact analysis, including the presence of curbing and other fixed objects. These new features, of themselves, do not directly impact or limit the average motorist's choice of operating speed. Moreover, these features are prevalent in urban and suburban areas where posted speed limits are determined by free-flowing speed analysis, rather than the statutory speed limits that generally govern rural roadways.

In rural areas, design speed limitations are represented by warning advisory speed signing which does not conflict with statutory speed signing. However, in suburban and urban areas operating with posted speed limits established to represent factors that do not directly impact motorist driving habits will often be lower than the speed resulting free-flowing posted speed analysis.

Should we allow design speed to influence (lower) posted speed limits rather than that established by measuring motorist behavior?

Dilemma: If we do, speed limit violations will increase accordingly. Enforcement emphasis will lead to a proportional increase in speeding citations, which will lead to motorist discontent and the accusations of speed traps.

If we don't, are we telling people they can drive at a speed higher than our "standards" indicate they should be traveling? Are we increasing our liability exposure due to our inability to articulate and then sell to the judicial system the reason(s) why these speeds (design and posted) should not be compared? Can we convince anyone that there is, at times, no correlation between design and posted (operating) speeds.

It seems to me that we can;t live with either alternative. If you agree, we collectively must change something. The most reasonable answer is to not use design features in design speed calculations that do not directly influence comfortable operating speed. As such, design speed should remain consistent with posted speeds established by measuring driver behavior.

Bottom Line: Design speed used for developing design consistency (and relativity) must closely represent the speed which conscientious free-flowing drivers will choose to travel.
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Author:Winkle, Stephen N. van
Publication:Public Works
Date:Jan 1, 1992
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