Setting and Posting Speed Limits
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| Figure 1. An improperly installed Speed Limit Sign (R2-1) |
Speed Limit Determination
RSA 265:60 sets speed limits for certain areas. The road in Figure 1 is a “rural residence district” with a statutory speed limit of 35 mph. RSA 265:63 also gives municipalities the authority to modify the statutory speed zones. It and federal laws (MUTCD, 2B.11), however, limit that authority by requiring engineering and traffic studies. A municipality must hire a consultant unless it has “sufficient staff” to conduct the investigation. That staff must have the time and expertise to consider the many factors in a speed limit study.
• Road characteristics, such as travel surface, shoulder condition, road alignment and sight distance, road and shoulder width, the number of lanes, and passing zones.
• Land use and numbers of driveways.
• The speeds of vehicles, and the “85th percentile speed” (85% of the traffic flows at or below the prevailing speed).
• Safe speed for curves and intersections.
• Parking practices and pedestrian activity.
• Vehicle volume, types, and characteristics.
• Weather and climate.
• Accident frequency and severity for the past 12 months.
Investigators must collect and analyze data for all the factors.
The Vermont Local Roads Program’s Setting Speed Limits has worksheets for data collection and analysis. (See the UNH T˛ Center website to view this booklet, or page 9 to order it and applicable RSAs.)
Investigators should also consider Advisory Speed Plaques (W13-1). They tell drivers that a lower speed might be necessary at curves, turns, intersections, and other local conditions. Although not enforceable, they add emphasis and information to other warning signs. The MUTCD requires that advisory speeds be determined by an engineering study.
A technically determined speed limit is usually the 85th percentile speed. Especially in modern residential developments, that speed might be dangerous to pedestrians and bicyclist. Lowering speed limits, however, are seldom the best solution. More than 15% of the drivers will then exceed speed limits. Many residents and pedestrians will have a false sense of security. Traffic calming techniques are usually a better solution. (See “Taming Traffic” in Road Business, Summer 2001)
Speed Limit Sign Installation
The Speed Limit Sign (R2-1) informs motorists of speeds required by law. The MUTCD requires a Speed Limit Sign be installed
• Where one speed limit changes to another,
• Beyond major intersections and where necessary to remind drivers of the lawful speed,
• At municipal boundaries, and
• Where easily seen by all motorists.
In Figure 1, even without the R2-1 sign, the Horizontal Alignment/Intersection Sign (W1-10) is too low. The bottom of a single sign must be at least 5 feet above the near edge of the pavement. The height must be 7 feet where parking or pedestrian movement occurs. For two signs, mounting height to the bottom of the secondary sign may be one foot less.
The Institute of Traffic Engineers (ITE) recommends that agencies normally erect signs on individual posts. Grouping is appropriate when one sign supplements another. Grouping, if it does not confuse motorists, can reduce roadside obstructions and to safe money.
After the Figure 1 picture was taken, the Town decided that the W1-10/R2-1 grouping might be confusing. It replaced the speed limit sign with an Advisory Speed Plaque. It relocated the Speed Limit Sign past the intersection. This meets the general MUTCD standard for location: “Signs requiring different decisions by the road user shall be spaced sufficiently far apart for the required decisions to be made safely.”
Sources
MUTCD. Manual of Uniform Traffic Control Devices. 2002. Federal Highway Administration.
Revised Statutes Online. http://gencourt.state.nh.us/rsa/html/indexes/default.html.
Setting Speed Limits: A Guide for Vermont Towns. 1995. Vermont Local Roads Program.
“Setting Speed Limits on Local Roads.” 1999.Wisconsin Transportation Center.
Traffic Signing Handbook. 1997. Institute of Traffic Engineers.