Section 4: Speed Zone Design
Anchor: #i1002715Zone Length
The length of any section of zone set for a particular speed should be as long as possible and still be consistent with the 85th percentile speeds. These zone lengths should be shown on the strip map in miles to three decimal places. Where graduated zones on the approach to the city or town are at locations where speeds fluctuate, the speed zone should generally be 0.200 mile or more.
School zones are the exception to this rule and may be as short as reasonable in urban areas, depending on approach speeds. School zones in urban areas where speeds are 30 miles per hour or less may have school zones as short as 200 to 300 feet.
Anchor: #i1002730Transitions
The change in speed between two adjacent zones should not normally be greater than 15 miles per hour, because the change in speed would be too abrupt for driver observance. If adjacent 85th percentile speeds show an abrupt change of more than 15 miles per hour, a transition zone of approximately 0.200 mile or more in length should be used.
Anchor: #i1002740Urban Areas
Texas law states that the maximum speed limit through an urban district is 30 miles per hour, unless zoned otherwise by proper authority. Therefore, a reasonable and prudent speed limit should be determined and negotiated with the city and set by city ordinance or by Transportation Commission minute order. A section of highway in this category should be speed zoned by commission minute order only if all negotiations with the city have proved unsuccessful.
Anchor: #i1002750Directional Differences
The 85th percentile speeds may differ considerably by direction at some locations. Such conditions are usually caused by relatively heavy development on one side of the road. Next to the development, motorists will tend to drive slower because of interference from traffic to and from the development.
On divided highways, the zone speeds should conform to the 85th percentile speed even though this may require zoning for different speeds in opposite directions.
On undivided roadways, the zones in opposite directions should be the same for enforcement purposes.
Anchor: #i1002770Variation from 85th Percentile
The posted speed selected is the nearest value ending in 5 or 0. The final speed limit may be lowered or raised by as much as 5 miles per hour from the 85th percentile speed or trial-run speed (if 125 cars cannot be checked during the two- or four-hour speed check) determined by the study, based on the professional judgment of the supervising engineer. Only under special conditions would the zone speed vary further from the 85th percentile. Explanations of such conditions follow.
Different Results at Adjacent Speed Check Stations. If the 85th percentile speeds for adjacent speed check stations are approximately the same, they may be averaged to determine the zone speed. Any 85th percentile speed should not be included in such averages if it varies more than 7 miles per hour from the speed derived from the average.
Crash Rate Greater than the Statewide Average Crash Rate for Similar Types of Roadways. When establishing a speed within an existing zone on the state highway system, the speed limit may be reduced by up to 12 miles per hour below the 85th percentile speed if the crash rate in the section of the roadway is greater than the statewide average crash rate for similar roadways. The latest speed study that has been performed on the roadway should be evaluated to determine if the information is still relevant based on the roadway traffic and characteristics. If so, then this can be used as the basis for lowering the speed limit. If the existing speed study is not valid, the district has three options:
- perform a full 85th percentile speed study
- perform an 85th percentile study at one or more locations within the segment
- perform a trial run speed study within the segment.
After determination of the 85th percentile speed, the following factors should also be considered to determine the total speed reduction up to 12 mph:
- narrow roadway pavement
- horizontal and vertical curves
- high driveway density
- lack of striped, improved shoulders
- crash history within the speed zone.
The final decision on the amount of variation should be based on the engineering judgment of the supervising engineer. Under this process, a strip map is not required. All other speed zoning rules within this manual apply for this section.
TRF will routinely provide information to the districts for roadways that meet the criteria for using this process.
Light Traffic Volumes. At locations where traffic volumes are light and 125 cars cannot be checked in the two or four hours that the speed check station is operated, the 85th percentile speed may not be reliable. Trial runs need to be made and documented in the study. (“Trial runs” are defined and explained later in this section.) Trial runs may be documented using the Summary of Trial Run for Speed Zones (TxDOT Form 1929),to supplement a strip map. (The form is available via hyperlink — click on the form number above — or from the Traffic Operations Division.) Figure 3-10, Example of completed Summary of Trial Run for Speed Zones, shows an example of a completed Summary of Trial Run for Speed Zones.
Legislative or Congressional Action. Notwithstanding the volume of traffic, if legislative or congressional action results in the immediate increase in statewide maximum legal speed limits, then reasonable and prudent speed zones may be established by trial runs and engineering judgment in lieu of other speed check procedures provided in this manual. (“Trial runs” are defined and explained later in this section.) Trial runs may be documented using the Summary of Trial Run for Speed Zones (TxDOT Form 1929) instead of a strip map. (The form is available via hyperlink — click on the form number above — or from the Traffic Operations Division.) Figure 3-10, Example of completed Summary of Trial Run for Speed Zones, shows an example of a completed Summary of Trial Run for Speed Zones. Speed zones established through this process should be rechecked in accordance with the procedure in Section 5 of this chapter.
Provisional Traffic and Engineering Investigation Requirements. When increasing the speed limit from 70 to 75 miles per hour as authorized by the legislature, the speed zone study may be limited to the determination of the 85th percentile speed at one or more speed check locations within the established speed zone. Because the boundaries of the speed zone have been established for the 70 mile per hour zone, a strip map is not required for the increase.All other speed zoning rules within this manual apply to the provisional traffic and engineering investigations.
Additional Roadway Factors. The posted speed limit may be reduced by as much as 10 miles per hour (12 miles per hour for locations with crash rates higher than the statewide average) below the 85th percentile speed or trial-run speed (if 125 cars cannot be checked during the two- or four-hour speed check), based on sound and generally accepted engineering judgment that includes consideration of the following factors:
- narrow roadway pavement widths (20 feet or less, for example)
- horizontal and vertical curves (possible limited sight distance)
- hidden driveways and other developments (possible limited sight distance)
- high driveway density (the higher the number of driveways, the higher the potential for encountering entering and turning vehicles)
- crash history along the location
- rural residential or developed areas (higher potential for pedestrian and bicycle traffic)
- lack of striped, improved shoulders (constricted lateral movement).
Local public opinion may also be considered on farm-to-market and ranch-to-market roads without improved shoulders (Transportation Code, Section 545.3535(b)).
The final decision on the amount of variation from the 85th percentile speed for a specific roadway should be based on the engineering judgment of the supervising engineer. If additional roadway factors are used to reduce the speed limit, include the factor or factors on the speed zone strip map.
Speed limits should not be posted more than 10 miles per hour (12 miles per hour for locations with crash rates higher than the statewide average) below the 85th percentile or trial-run speed (if 125 cars cannot be checked during the two- or four-hour speed check), since unreasonably low speed limits have not been shown to be an effective way to control speeding. Allowing too great a variation would risk losing motorist respect for speed limits and traffic control devices.
Anchor: #i1002860Blanket Lowering of Maximum Speed Limits
A blanket lowering of maximum speed limits may be justified:
- during either state or national emergencies or disasters, such as war or energy crisis, where an authoritative study indicates that a reduction of speeds will result in a significant reduction in the consumption of fuel and energy and will promote fuel and energy conservation
- to avoid non-compliance with direct requests from the federal government to lower the statewide maximum speed limit to a speed equal to or below the national speed limit.
Trial Runs
A “trial run” is a drive through the speed zoned section of roadway at the chosen speed(s) to determine if the speeds are appropriate for the area.
After the 85th percentile speeds and zone lengths have been selected, several trial runs should be made through the area in both directions driving at the selected speeds. This should show any irregularities in the zoning which need correction.
Documentation. Trial runs may be documented using the Summary of Trial Run for Speed Zones (TxDOT Form 1929) to supplement a strip map. (The form is available via hyperlink — click on the form number above — or from the Traffic Operations Division.) Figure 3-10 Example of completed Summary of Trial Run for Speed Zones, shows an example of a completed Summary of Trial Run for Speed Zones.
Anchor: #i1002902Location of Regulatory Speed Limit Signs
Since speed zones are legally described to the nearest thousandth of a mile (5 feet), regulatory speed limit signs should be located within approximately 5 feet of the actual reference marker or milepoint defined in the minute order or city ordinance. Therefore the locations of regulatory speed zones tied to speed changes should be examined carefully to ensure that signs can be erected within the 5 feet variation. If adherence to the 5-feet variation is not possible, the SPEED ZONE sign should be placed as close to the actual location defined in the minute order or city ordinance as practical. For example, if the reference marker or mile point is located at an intersection, the regulatory speed limit signs should be located in accordance with standard procedures for placement of departure signing.
Figure 3-10. Example of completed Summary of Trial Run for Speed Zones