4.2 Speed
Speed is one of the most important factors considered by travelers in selecting alternative routes or transportation modes. In addition to capabilities of the drivers and their vehicles, the speed of vehicles on a road depends upon five general conditions: the physical characteristics of the roadway, the amount of roadside interference, the weather, the presence of other vehicles, and speed limitations established by law or by traffic control devices. Although any one of these factors may govern travel speed, the actual travel speed on a facility usually reflects a combination of factors.
The objective in design of any engineered facility used by the public is to satisfy the public’s demand for service in an economical, efficient, and safe manner with low crash frequency and severity. The facility should accommodate nearly all demands with reasonable adequacy and should only fail under severe or extreme traffic demands. Because only a small percentage of drivers travel at extremely high speed, it is not practical to design facilities for these speeds. On the other hand, the speed chosen for design should not be based on the speeds drivers use under unfavorable conditions (such as inclement weather), because the roadway would then be inefficient, might result in additional crashes under favorable conditions, and would not satisfy reasonable public expectations for the facility.
There are important differences between design criteria applicable to low-speed and high-speed designs.
For design purposes, the following definitions apply:
- Low-speedis 45 mph and below; and
- High-speedis 50 mph and above.
4.2.1 Operating Speed
Operating speed is the speed at which drivers are observed operating their vehicles during free-flow conditions.
The 85
associated with a particular location or geometric feature.th
percentile of the distribution of observed speeds is the most frequently used measure of the operating speedThe following geometric design and traffic demand features may have direct impacts on operating speed:
- Horizontal curve radius;
- Grade;
- Access density;
- Median treatments;
- On-street parking;
- Signal density;
- Vehicular traffic volume;
- Lane widths;
- Sight distance; and
- Pedestrian and bicycle activity.
4.2.2 Running Speed
The speed at which an individual vehicle travels over a highway section is known as its running speed. Running speed is calculated by dividing the length of the highway section by the time for a typical vehicle to travel through the section. For extended sections of roadway that include multiple roadway types,
the average running speed is the most appropriate measure for evaluating level of service (LOS) and road user costs.
The average running speed is the sum of the distances traveled by vehicles on a highway section during a specified period of time divided by the sum of the travel times.The average running speed on a given roadway varies during the day, depending primarily on the traffic volume. Therefore,
when reference is made to a running speed, clearly state whether this speed represents peak hours, off-peak hours, or an average for the day.
It is most appropriate to use peak and off-peak running speeds in design and operation. Average running speeds for an entire day should be reserved for economic analyses. The effect of traffic volume on average running speed can be determined using the procedures of the
.4.2.3 Design Speed
Design speed is a selected speed used to determine the various geometric design features of the roadway.
The selected design speed should be logical with respect to the anticipated operating speed, topography, adjacent land use, modal mix, and functional classification of the roadway.
Design speed is different from other controlling criteria in that it is a design control, rather than a specific design element. The design speed establishes the range of design values for many of the other geometric elements of the highway.
In selection of design speed, every effort should be made to attain a desired combination of safety, mobility, and efficiency within the constraints of environmental quality, economics, aesthetics, and social or political impacts.
A roadway of higher functional classification may justify a higher design speed than a lower classified facility in similar terrain. A low design speed should not be selected where topography and other factors are such that drivers are likely to travel at high speeds.
Some factors to consider when choosing a design speed include:
- The expectations of drivers which are closely related to traffic volume conditions;
- Potential traffic conflicts; and
- Terrain features.
4.2.4 Target Speed
The working definition for “target speed” is the operating speed that the designer
intends
for drivers to use. In mixed-use urban areas, the target speed
is the highest speed at which vehicles should operate on a low-speed thoroughfare in a specific context, consistent with the level of multimodal activity generated by adjacent land uses, to provide both mobility for motor vehicles and a desirable environment for pedestrians, bicyclists, and public transit users. In general, the target speed for rural roadways should be on the higher end of the design speed range.The target speed approaching roundabouts, or some other alternative types of intersections, should be properly stepped down to accommodate the desired lower target speed within the intersection. See
Alternative Intersections for additional information on when target speeds should be stepped down as drivers approach these types of intersections.
The findings of
, which is to be completed in late 2024, will be used as an additional resource to provide recommendations in the roadway design process.
In general, the Target Speed for rural roadways should be on the higher end of the Design speed range. In general, for urban roadways, consider starting with Target speeds on the lower end of the Design speed range.
4.2.5 Posted Speed
Posted speed refers to the maximum speed limit posted on a section of highway. The
states that the posted speed should be based primarily upon the 85th percentile speed when adequate speed samples can be secured. Speed zoning guidelines permit consideration of other factors such as:
- Roadside development;
- Road and shoulder surface characteristics;
- Public input; and
- Pedestrian and bicycle activity.