| are in affect 24 hours a day. Other situations may not lend themselves to such transitions zones, and should be left up to engineering judgment. (B) Sign design. (i) The basic sign design for a school transition speed limit shall be the same as that used for a regular school zone speed limit sign. (ii) Where the department is responsible for signing school zone speeds and school transition speed zones, the "School Speed Limit XX When Flashing" signs shall be used. (d) Speed zone design. (1) Zone length. (A) 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 are at locations where speeds fluctuate, the speed zone should generally be 0.2 mile or more. (B) School zones are the exception to this rule and may be as short as reasonable in urban areas, depending on approach speeds. (i) 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. (ii) Where speeds exceed 40 miles per hour, the minimum school zone length should be 1,000 feet to allow for normal deceleration. (2) Transitions. (A) 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. (B) If adjacent 85th percentile speeds show an abrupt change of more than 15 miles per hour, a transition zone of approximately 0.2 mile or more in length should be used. (3) Urban areas. Texas law states that the maximum speed limit through an urban district is 30 miles per hour, unless zoned otherwise by proper authority. A reasonable and prudent speed limit should be determined and negotiated with the city and set by city ordinance or resolution or by 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. (4) Directional differences. (A) 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. (B) 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. (C) On undivided roadways, the zones in opposite directions should be the same for enforcement purposes. (5) Variation from 85th percentile. (A) 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 (performed if 125 cars cannot be checked during the two or four hour speed check) 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. (i) 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. (ii) Crash rate greater than average. On a section of highway having a crash rate greater than the statewide average crash rate for the same type of roadway section, the zone speed may be as much as 12 miles per hour lower than the 85th percentile speed. This should be considered more as an exception than as a rule, and should be done only when enforcement agencies will assure a degree of enforcement that will make the speed zone effective. (iii) 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. (iv) 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 subchapter. Speed zones established through this process should be rechecked in accordance with the procedure in subsection (e) of this section. (v) 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 (performed 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: (I) narrow roadway pavement widths - 20 feet or less, for example; (II) horizontal and vertical curves - possible limited sight distance; (III) hidden driveways and other developments - possible limited sight distance; (IV) high driveway density - the higher the number of driveways, the higher the potential for encountering entering and turning vehicles; (V) crash history along the location; (VI) rural residential or developed areas - higher potential for pedestrian and bicycle traffic; and (VII) lack of striped, improved shoulders - constricted lateral movement. (B) Local public opinion may also be considered on farm-to-market and ranch-to-market roads without improved shoulders (Transportation Code, §545.3535(b)). (C) 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. (D) 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 (performed 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. (6) Blanket lowering of maximum speed limits. Blanket lowering of speed limits may be justified to avoid non-compliance with direct requests from the federal government to lower the statewide maximum speed limit. (7) Trial runs. (A) For the trial run, an average passenger vehicle that is representative of most vehicles on the highway and a reasonably competent driver should be selected. (B) 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. (8) Location of regulatory speed limit signs. (A) 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 or resolution. (B) 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 or resolution as practical. For example, if the reference marker or milepoint is located at an intersection, the regulatory speed limit signs should be located in accordance with standard procedures for placement of departure signing. (e) Rechecks of speed zones. (1) Introduction. (A) The basic data on which speed zones are established are subject to change when conditions change, and established speed zones must not be considered permanent. (B) Physical improvements to the roadway, increased roadside development, and heavy increases in traffic volumes justify a recheck of speeds to determine whether the 85th percentile speed has changed enough to require a change in the zone speeds. (2) Frequency of rechecks. Cont'd... |
