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FHWA Highway Safety Programs

CHAPTER 7. MANAGING TRAFFIC SPEED

Traffic speeds involve a complex set of interactions between engineering, legal and driver performance factors. Currently, knowledge of speed behavior is limited. Although the research literature contains a variety of speed prediction models for rural two-lane highways and low-speed urban streets, accurate speed prediction models for other road and street types is limited. As such, the ability to accurately predict speeds on all road and street types does not exist. Similarly, there is no reliable guidance on how to attain specific operating speed characteristics (e.g., mean, 85th percentile, speed deviation) and speed relationships (e.g., between 85th percentile and design speeds) during the geometric design process. Until this type of information is implemented, safety can be improved through strategies that result in better geometric designs and infrastructure conditions, more credible and effective speed control and targeted enforcement. Speed management is a strategy for controlling speed through a comprehensive, interdisciplinary and coordinated approach that encompasses behavioral, enforcement and engineering elements.

IMPROVING THE DESIGN PROCESS

Once constructed, transportation infrastructure is enduring. Roads and streets are public investments that establish spatial arrangements for community development and economic activity. Alterations may be costly and disruptive. Since the consequences of geometric design are significant and long-lasting, decisions should be deliberate. Geometric design is one potential influence on traffic speeds. Comprehensive and reliable speed prediction models do not exist for all road and street types. However, some information has been developed on geometry-speed relationships. The relationships shown in figure 18 were developed from data for horizontal curves on rural highways. Studies of similar scopes have reached similar findings. A reasonable conclusion from these studies is that drivers will operate as close to their preferred speed as possible, which may be below or above the inferred design speed. It is also reasonable to deduce that restrictive geometric features, especially tight horizontal curves, will tend to restrain operating speeds albeit not to the level of the inferred design speed.

The Green Book recommends that anticipated operating speed be considered in designating the design speed. The strong influence of driver desire and expectations on operating speed should be recognized in that determination. Expectations are formed, in part, on the function of the facility within the network.

Improving design consistency is another area of speed-related improvement. Desirably, specific features and locations along a travel route should not require unexpected speed reductions. Isolated speed-restrictive features are likely to violate a driver’s expectation that has developed from conditions encountered previously. Developing designs that accommodate a fairly uniform speed over an extended distance are preferable to those that will result in substantial speed fluctuations. Relative to driver expectancy, geometric features should transition from a higher to lower design speed gradually. As an example, long tangents (with an infinite inferred design speed) between tight curves should be avoided.

The Interactive Highway Safety Design Model (IHSDM) is a suite of software packages being developed by the FHWA. The IHSDM is planned as an integrated system of modules for evaluating the operational and safety effects of highway geometric design alternatives. The IHSDM Design Consistency Module evaluates the consistency of a design relative to various speed measures.

Graph. A line graph shows the mean and 85th percentile relationship between inferred driving speed and operating speed on horizontal curves.

Figure 18. Mean and 85th percentile speeds versus inferred design speed on horizontal curves.

SETTING AND ENFORCING RATIONAL SPEED LIMITS

The management of speed through appropriate speed limits is an essential element of highway safety. Appropriate speed limits are a prerequisite for effective and sustainable speed management. Speed limits should reflect the maximum reasonable speed for normal conditions. Speed limits should be accepted as reasonable by most drivers. Not all drivers will conform to reasonable speed limits. In essence, speed limits separate high-risk and reasonable behavior. If lower speed limits are desired, then engineering and other measures should be implemented that reduce speeds to a level that would support a lower limit.

NHTSA and FHWA jointly support efforts to demonstrate and evaluate an integrated "three E’s" (engineering, enforcement, education) approach to the management of speed and crash risk. Rational speed limits are established on the basis of an engineering study of prevailing speed and other factors such as pedestrian activity and crash history. The 85th percentile speed is typically used as a starting point for setting a rational limit but it may be set as low as the average speed based on other factors, such as those listed previously in table 3. Once the speed limits are appropriately set and the judiciary informed, a program of strict enforcement with a low tolerance for speeds exceeding the limits is combined with public information and education explaining the purpose of the revised limits and the consequences for violators. Evaluation of program effectiveness is a critical element of the demonstrations.

SPEED LIMIT SOFTWARE

With so many technical and legal factors involved, government agencies may need help in determining appropriate speed limits. This type of assistance is available through the web-based expert speed zoning software advisor USLIMITS. This software provides an objective and consistent process for incorporating the various factors that may be considered in the decision making process according to the MUTCD but for which no guidance is provided. The speed zone expert system was adapted from similar expert systems used by most Australian state road authorities but modified to reflect elements of speed setting philosophy used in the U.S.

The expert system recommends a speed limit for a section of road based on road function, roadside development, operating speeds, road characteristics and other factors required to determine appropriate speed limits in speed zones. The system also warns users of issues that might require further investigation and engineering judgment. USLIMITS provides a screen report and a more detailed print report. USLIMITS will be of particular use to small communities and agencies that lack experienced traffic engineers. For experienced traffic engineers, it can provide a second opinion and increase confidence in speed zoning decisions.

For further information on USLIMITS see http://www.uslimits.com

MITIGATION MEASURES

Speed limits are not the only tool that agencies can draw on to manage operating speeds. In fact, as discussed previously speed limits usually have a limited effect on operating speeds. Roadway geometry and the frequency of enforcement also play a role in driver judgments and choices regarding speed. A number of proven and promising speed management practices and technologies are available. The suitability of each as an element in an agency’s speed management and safety program should be evaluated based on the community, legal and transportation contexts.

Advisory Speed Posting

Speed limits should not be lowered to reflect an isolated restrictive element. This practice tends to reduce the credibility of speed limits. When a speed lower than the speed limit is appropriate for a particular location, the use of an advisory speed plaque and associated traffic control devices should be considered. The MUTCD and other publications, such as state DOT procedures and traffic manuals, provide guidance on the use of Advisory Speed signs.

In the Notice of Proposed Amendments (NPA) to the MUTCD, the FHWA proposes that advisory speeds be determined by engineering study. The NPA also indicates that the advisory speed should be determined based on free-flowing conditions and , because changes in conditions (e.g. roadway geometrics and sight distance) might affect the advisory speed, each location should be periodically evaluated when conditions change.The MUTCD also provides guidance on advanced placement of signs prior to the horizontal curve as well as supplemental distance plaques where there is an area with continuous roadway curves. Figure 19 shows an example of a horizontal alignment change (i.e., curve) combined with advisory speed plaque.

Photo. A rural roadway is shown with a horizontal curve warning sign on the shoulder. The sign consists of two separate components; the top is a yellow diamond with a black curve arrow and the second is a smaller square plaque displaying 35 m.p.h.

Figure 19. Horizontal curve warning sign with speed advisory plaque.

The NPA proposes the following among established engineering practices to determine the recommended advisory speed for a horizontal curve:

  • An accelerometer that provides a direct determination of side friction factors.
  • A design speed equation (see Equation 2, page 13).
  • A ball-bank indicator using the following criteria:
    • 16 degrees for speeds of 20 mph or less.
    • 14 degrees for speeds of 25 and 30 mph.
    • 12 degrees for speeds of 35 mph and higher.

The ball-bank indicator readings are similar to those contained in the AASHTO Green Book (see Table 4).

Two other methods of determining a curve advisory speed have been recently outlined in a Horizontal Curve Signing Handbook prepared by the Texas Transportation Institute (19). The two methods are referred to as the direction method and the compass method. Each is discussed briefly below.

The direct method is simple to apply, requiring minimal time and cost. The first step is to record 125 or more free-flow passenger cars in each direction of travel and compute the mean speed separately for each direction. The speed measurements should be measured within the middle third of the curve. Free-flow vehicles are those with at least three seconds of space between leading and following vehicles. The second step is to multiply both average speeds from the first step by 0.97, which approximates the average truck-speed. The advisory speed for each direction is calculated by adding 1 mph to the average truck speed and then round down to the nearest 5 mph increment, unless the value ends in a 4 or 9. If the value ends in a 4 or 9, then it should be rounded up to the next 5 mph increment. This value should be confirmed by driving the curve at the advisory speed.

The compass method requires field data, which are then used as input in a truck speed prediction model. The average truck speed from the model is recommended as the advisory speed. To be accurate, the method requires that the curve length be at least 200 feet long and the curve deflection angle should be at least 15 degrees. In the first step of the procedure, the analyst must measure the curve deflection in the direction of travel, heading at the 1/3 point of the curve, ball-bank reading of curve superelevation at the 1/3 point of the curve, the length of curve between the 1/3 and 2/3 points of the curve, the heading at the 2/3 point of the curve, and the posted speed limit or 85th percentile speed on the tangents near the curve. A digital compass, distance-measuring instrument, and ball-bank indicator are required to make the field measurements. In the second step of the procedure, the Texas Curve Advisory Speed (TCAS) software is used to compute a "rounded speed advisory." This speed should be confirmed by driving the curve at the recommended advisory speed.

The intent of the compass method is to improve the process for determining curve advisory speeds – the MUTCD indicates that advisory speeds may be based on the 85th-percentile speed of free-flowing traffic, the speed determined by an engineering study, or the speed corresponding to a 16-degree ball-bank indicator reading (16). A ball-bank indicator is normally attached to a vehicle such that its physical presence will not change while driving through a horizontal curve. The ball-bank indicator reading is a function of the lateral acceleration and roll rate of a vehicle. From this, a speed at which drivers will become uncomfortable while traversing a horizontal curve can be determined. The MUTCD ball-bank indicator method assumes a constant friction threshold for all speeds, which is not necessarily applicable to highway geometric design. The compass method accounts for variable side-friction thresholds for different speeds.

Specific details concerning the direct and compass methods to set curve advisory speeds can be found in Bonneson, et al. (19), or at the following link: (http://tti.tamu.edu/documents/0-5439-P1.pdf).

Improving Friction on Roadway Surfaces

Friction is needed to drive, brake and corner a vehicle. Forces are transmitted between the vehicle and road through friction at the tire-road interface. The characteristics of driving maneuvers (e.g., turning radius and speed) influence the frictional demand. The available friction is a characteristic of roadway material and vehicle tire properties. It is possible but undesirable for the demand friction to exceed available friction. Think about a car trying to stop on an ice-covered section of road.

The available friction needed for both turning and stopping decreases with increasing speed. However, as vehicle speeds increase, friction demand increases. The maximum design values for friction adequately accommodate the design speed over a wide range of conditions, including poor conditions (i.e., wet/icy pavement, worn tires, smooth road). However, at some level of speed, the available friction will be exceeded. High speeds, alone or in combination with other factors, increases the probability that available friction will be exceeded by the demand. Adverse environmental conditions (e.g., wet weather) increase the frequency of this condition and one of its more common results, skidding.

The need for friction is a major consideration in the selection and application of materials and treatments for all pavements. Many transportation agencies also have programs to increase skid-resistance at particularly vulnerable locations, such as curves and intersections. Examples of treatments used by agencies include: bituminous surface treatments, chip seals, grooving and microtexturing.

Bituminous surface treatments should include aggregate gradations that create voids in the material. This is intended to increase surface drainage as well as improve skid resistance. Agencies should first repair all major surface defects and then apply a bituminous tack coat. Finally, a bituminous surface treatment can then be overlaid onto the roadway surface. Figure 20 is an example of a bituminous surface treatment.

Photo. A roadway is shown at the approach to a curve. A difference can be seen in the color of the road surface in the curve.

Figure 20. Example bituminous surface treatment.

Grooving the surface of a horizontal curve can be completed using a portable milling machine. An accepted technique is to use carbide-tipped flails to install grooves 3/16 to 3/8 inch wide and 5/32 to 5/16 inch deep, with 8 grooves per foot on a random spacing (20). Longitudinal grooves have been shown to increase directional control of the vehicle while transverse grooves are most effective at locations where vehicles are making stops.

Microtexture is the small-scale roughness that is related to the fine aggregate in a mix (21). Microtexture relates to the size of small aggregate and the surface roughness of larger aggregate. As pavement wears and weathers over time, microtexture can be lost. This can be avoided by using high-quality, skid resistant aggregate. However, microtexture loss can be restored through pavement overlays, such as open-graded friction courses. Diamond grinding Portland Cement Concrete (PCC) increases the microtexture by dislodging sand particles in the mix. The longitudinal direction of grinding also increases directional stability of vehicles.

Chip sealing is the process of sealing an asphalt roadway surface with a polymer-modified asphaltic emulsion and crushed aggregate (22). The emulsion seals the cracks in the roadway surface, keeping moisture out of the surface and base. The crushed aggregate provides for increased friction as well as an all weather wearing surface.

Speed Display Signs

Speed display signs measure the speed of approaching vehicles, typically with radar, and display the measured speed. LED (light emitting diodes) is a common display technology. The signs may be mounted on trailers to increase portability or on fixed support systems. An example dynamic speed display sign is shown below in figure 21. Many of the signs also display the applicable speed limit, creating a direct comparison for drivers. Speed display signs were originally used in connection with temporary conditions, such as works zones. More recently, agencies have deployed and begun to evaluate the effectiveness of these signs in reducing speeds at permanent, speed-sensitive locations. One study (23) investigated installations at three locations and found various levels of speed-reduction effectiveness. For example, a speed display sign installed at the entry of a school zone led to a 9-mph reduction in the average speed. Smaller reductions were found at other locations.

Photo. A typical speed limit sign posting 35 is shown with a digital display underneath. The display says, “your speed” and then has a dynamic number – 46 in this case.

Figure 21. Example dynamic speed display sign.

On the basis of this evaluation, the researchers offered the following insights on how various factors affect the effectiveness of speed display signs:

  • More effective if perception of regular enforcement (and threat of citation) exists at site.
  • More effective if the sight distance to the treated condition is less than decision sight distance.
  • More effective where only one lane exists per direction.
  • More effective if used with other information "indicators" of a need to reduce speed (school speed limit beacons, signal change in area of speed display sign warning beacons, etc.).
  • More effective if the speed display sign is used to support a regulatory speed limit (as opposed to an advisory speed limit).
  • More effective if the overall information system at the location does not overwhelm the speed display sign.

Speed display signs are available from a number of vendors.

Traffic Calming

Traffic calming is a term used to describe a set of techniques, consisting mostly of physical features, to affect vehicle operations on one or more streets to improve the street environment for other users (i.e., those not using motorized vehicles). Speed reduction is one of several traffic calming objectives. The specific traffic calming measures selected for application at a particular location should correspond with the unique conditions of the location and the objectives, since not every speed calming technique is appropriate for every roadway. Also, including such measures can result in drivers slowing at the speed calming feature, and speeding between them. Some of the measures that have been employed to reduce vehicle speeds include:

  • Speed humps.
  • Speed tables.
  • Raised intersections.
  • Traffic circles.
  • Modern roundabouts.
  • Chicanes.
  • Lateral shifts.
  • Realigned intersections.
  • Lane narrowing.

As noted above, traffic calming encompasses many issues other than speed management. This short summary, which emphasizes speed considerations, should not serve as the basis for traffic calming decisions. The Institute of Transportation Engineers (ITE) and the FHWA have brought together the experiences and practices of many individuals and agencies in the informational report Traffic Calming: State of the Practice (24). The report is for informational purposes and does not recommend the preferred or appropriate treatment for a particular location or set of conditions. The report includes more detailed information on the installation and reported effectiveness of specific measures and devices.

Variable Speed Limits

Variable speed limits are speed limits that change based on road, traffic, or weather conditions. At a particular time and place, the applicable speed limit reflects some of the same factors a prudent driver also considers. Examples include the effects of reduced visibility and slick road conditions (which may increase the required stopping distance). Improving the consistency between a responsible driver’s speed selection and the speed limit may help to restore speed limit credibility and improve safety.

The experience base with variable speed limits in the U.S. is limited. Quite a few agencies use changeable message signs to display speeds that vary with prevailing conditions but nearly all of these systems display advisory speeds. The underlying algorithms and display technologies may be very well developed but the enforcement and legal sanctions that define speed limits are not present. The most familiar U.S. application of variable speed limits is in school zones, where the speed limit changes in relation to the school schedule. Although not common, states are beginning to experiment with systems that could lead to variable speed limits in work zones.

The movement toward variable speed limits in the U.S. is in its early development phase. The legal doctrine and enabling legislation will evolve through legislative action and judicial review and interpretation.

Improve Sight Distance

AASHTO stopping sight distance values were revised in 2001 due to the different parameters of the changing vehicle fleet. Since the new recommended stopping sight distance values are greater than the old minimum stopping sight distance criteria (although less than the requirement for desirable stopping sight distance), this has resulted in many roadways falling outside the current range of criteria, even though the stopping sight distance design criteria was met at the time. It has been published that "moderate reductions in minimum sight distance do not appear to be a safety problem (9)." Stopping sight distance profiles can show a designer where sight restrictions occur, and the amount of sight distance that is actually available at that location. Figure 22 shows an example sight distance profile that was produced from the IHSDM.

The first mitigation measure to increase available sight distance is to lengthen crest vertical curves. This measure, however, can be quite costly and can have significant impacts to adjacent land uses. If severe sight distance limitations exist, improvements can be made even if the minimum SSD is not met. Other improvements that can be made are to remove objects that are within the sight limited area as well as increase crash avoidance areas through lane widening or shoulder widening.

The available sight distance along horizontal curves and at intersections can be improved by controlling vegetation. Vegetation can decrease the sight distance available to traffic control devices as well as decrease sight distance on the inside of horizontal curves. Another mitigation measure for inadequate sight distance on an intersection leg is to install warning flashers on the approach leg.

Graph. An example of a stopping sight distance profile is shown.

Figure 22. Example stopping sight distance profile from the IHSDM.