Section 3: Factors Affecting Safe SpeedAnchor: #i1001829
This section discusses various factors influencing drivers and their perception of the safe speed at which to operate a vehicle. Because so many variables affect the safe operating speed of vehicles, it is not practical to consider each individually. These factors should be considered as a whole and weighed accordingly.Anchor: #i1001839
Design and Physical Factors of the Roadway
The design and physical factors of the roadway place a definite limitation on the safe operating speed of vehicles. These factors include:
- horizontal and vertical curves
- hidden driveways and other roadside developments
- high driveway density
- rural residential or developed areas
- lack of striped, improved shoulders.
Chapter 5, Section 2, Curves and Turns and Chapter 5, Section 5, Descending Grades of Six Percent or Greater of this manual provides the methods that must be used to determine if a curve or an obstruction to sight distance requires an advisory speed restriction.
The effects of such factors as lane width, condition of surface, type and width of shoulders, frequency of intersections, and roadside development are not so easily measured. As a general rule, especially on tangents, these factors will be measured on the basis of prevailing speeds as determined by speed checks.Anchor: #i1001886
The mechanical condition of vehicles and their characteristics for accelerating, decelerating, stopping, and turning definitely affect safe speeds.
The body roll angle of different makes of cars and year models of the same make also affects the safe operating speed on curves.
Braking capabilities of different vehicles, such as passenger cars, buses, and various truck-trailer combinations, are obviously different, and it would generally not be practical to post safe speeds for each group.
Normally, the posted speed will be that for the passenger car.Anchor: #i1001911
The ability of different drivers varies from skillful to those who would not be able to pass an examination for an operator’s license if required to take one. It would not be wise to post safe speeds for drivers at either of the two extremes of abilities, so the selection of speeds to be posted will be aimed at the ability and performance of the average driver.
Average driver ability, of course, is considered in the form of perception — reaction time in the calculation of critical approach speeds to intersections, crosswalks, and locations with limited sight distance and in determining the posting distance for signs.Anchor: #i1001926
The presence of other vehicles on the highway — including those that may be entering, crossing, turning off, or parked — definitely affects critical speeds.
The frequency of pedestrians is likewise an important factor. This is especially true at intersections with limited sight distance and at approaches to crosswalks.
The speeds shall be posted for off-peak hour traffic on an average weekday for the purpose of the procedures outlined herein. This will require drivers to adjust their speeds to lower values at times of peak hour traffic at some locations.Anchor: #i1001946
Weather and Visibility
Speeds will normally be selected and posted for good weather conditions and dry pavement. Texas law, however, also provides for the posting of speeds for wet weather conditions.
Except in cases where the statewide maximum legal limits are posted, speeds will normally be posted on the basis of daylight speed values determined under good weather conditions.
When it can be shown that it is required during wet or inclement weather, a wet weather speed zone may be established by Transportation Commission minute order. The wet weather speed limit should be posted in addition to the regular posted speed zone. When appropriately signed, this wet weather speed limit will be effective during wet weather at any time during hours of daylight and darkness.Anchor: #i1010618
Accident Reconstruction Speed Limits
Transportation Code, Section 545.3561 gives municipalities and counties the authority to temporarily lower prima facie speed limits at the site of a crash investigation using vehicular accident reconstruction. The municipality or county must use a transportation engineering official with experience establishing speed limits. For a municipality, the authority applies to a highway or part of a highway in the municipality, including a highway in the state highway system. For a county, the authority does not apply to a road or highway in the state highway system.
In establishing the speed limit, the municipality or county is not required to conduct an engineering and traffic study or comply with other provisions of this subchapter. To set the temporary speed limit, the municipality or the county must:
- follow safety guidelines as developed by the department for setting regulatory construction speed limits in work zone areas,
- provide notice to the department district engineer in the district in which the accident reconstruction is occurring at least 48 hours prior to the speed reduction, and
- during the time that the accident reconstruction is being conducted, place and maintain temporary speed limit signs that conform to the Texas Manual on Uniform Traffic Control Devices (TMUTCD), temporarily conceal all other signs that permit higher speeds, and remove the temporary signs and concealments when the accident reconstruction is complete.
- Notification to the department district engineer should be documented using the Notification Speed Limit Reduction for Crash Reconstruction (TxDOT Form 2455). The form is available via hyperlink--click on the form number above--or from the Traffic Operations Division.
If a traffic lane will be closed to accommodate the reconstruction investigation, the municipality or county must follow all department rules and guidelines on lane closures.
The department may remove any temporary speed limit signs or concealments of speed limit signs that are not removed by the municipality at the conclusion of the accident reconstruction.