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:

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 ,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, 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 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, 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:

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.