(a) Overview.   (1) Engineering and traffic investigation. This section includes information concerning interim speed limits for a new or reconstructed highway and a description of how to conduct an engineering and traffic investigation as the basis for establishing a regulatory speed zone along a roadway. This investigation is commonly called a speed zone study.   (2) Interim speed limits for new or reconstructed highways.     (A) An interim speed limit for a new or reconstructed highway shall be determined and posted before the highway is opened to traffic.     (B) To set an interim speed limit, a traffic and engineering investigation will be conducted.     (C) The traffic and engineering investigation will include a review of:       (i) the statutory prima facie speed applicable to the highway;       (ii) the design speed applicable to the highway; and       (iii) a trial run speed study for the highway.     (D) To set an interim speed limit at a speed that is less than the prima facie speed applicable to the highway, a commission minute order or a city ordinance setting the interim speed limit is required.     (E) Warning signs and advisory speed signs may be used on a highway with an interim speed limit to alert drivers to any hazards.     (F) When traffic speeds have stabilized on the highway, an 85th percentile speed study will be conducted under subsection (b) of this section. The interim speed limit will be used on the highway until the maximum speed for the highway is determined following the conclusion of that investigation.   (3) Scope of study.     (A) The speed zone study should cover the entire length of a potential zone, even though an analysis of the data may later indicate that the actual limits of the area that requires zoning are less than the limits of the potential zone.     (B) A speed zone study consists of the following principle areas:       (i) determining the 85th percentile speed;       (ii) crash study;       (iii) developing of strip maps;       (iv) speed zone design; and       (v) rechecks of speed zones. (b) Determining the 85th percentile speed.   (1) General concepts.     (A) The maximum speed limits posted as the result of a study should be based primarily on the 85th percentile speed, when adequate speed samples can be secured.     (B) Speed checks should be made as quickly as possible, but it is not necessary to check the speed of every car. In many cases, traffic will be much too heavy for the observer to check all cars.   (2) Speed checks for new or reconstructed highways. Speed checks on new or reconstructed highway sections should not be performed until it is apparent that the traffic speeds have stabilized.   (3) Operation of speed check stations.     (A) Normal speed checks should:       (i) be made on average week days during off-peak hours under normal traffic conditions;       (ii) be made under favorable weather conditions;       (iii) include only "free floating" vehicles (see subparagraph (B) of this paragraph);       (iv) include a minimum of 125 cars in each direction at each station; and       (v) be discontinued after two hours using a radar or four hours if performed by a traffic counter that classifies vehicles by type, even if 125 cars have not been timed.     (B) The vehicles checked should be only those in which drivers are choosing their own speed ("free floating").       (i) When a line of vehicles moving closely behind each other passes the speed check station, only the speed of the first vehicle should be checked, since the other drivers may not be choosing their own speed.       (ii) Cars involved in passing or turning maneuvers should not be checked, because they are probably driving at an abnormal rate of speed.     (C) Trucks and buses should be recorded separately and should not be included as part of the 125-car total.   (4) Location of speed check stations.     (A) A complete picture of speeds in an area can only be obtained through the proper location of speed check stations. Ideally, speed checks at an infinite number of locations would be desirable. However, since this is not practical, speed check stations must be strategically located to show all the important changes in prevailing speeds.     (B) In urban areas and on approaches to cities, speed check stations:       (i) should generally be located at intervals of 0.25 mile or less if necessary to insure an accurate picture of the speed pattern;       (ii) should be located midway between signals or 0.2 miles from any signal, whichever is less, to ensure an accurate representation of speed patterns;       (iii) should take into account the locality and the uniformity of physical and traffic conditions;       (iv) may be determined by trial runs through the area if volumes are too low or if a recheck of speeds is all that is needed; and       (v) should be checked midway between interchanges on the main lanes of expressways and freeways.     (C) In rural areas, speed check stations:       (i) may be at intervals greater than 0.25 mile, as long as the general speed pattern is followed;       (ii) may only be necessary at each end and the middle point if the characteristics of the roadway are consistent throughout the entire section; and       (iii) may be determined by trial runs through the area if the characteristics of the roadway are consistent throughout the entire section and a speed check in that section indicates that 125 vehicles cannot be checked in the two hours using a radar or four hours if performed by a traffic counter that classifies vehicles by type.   (5) Measuring speeds.     (A) Radar speed meters which operate on the radar principle are normally used for making speed checks. These devices operate from the power of an automobile battery and give direct readings of vehicle speeds in miles per hour which are accurate to within 2 miles per hour.     (B) New technologies may be used in determining vehicular speeds for use in calculating 85th percentile speed if the measured speeds are accurate to within 2 miles per hour and the gap between vehicles is 3 seconds or greater. Examples of new technologies are counter-classifiers with the capability of classifying vehicles, determining vehicular speeds, and differentiating the gap between vehicles. These devices may include video imaging, tube counters, magnetic counters, inductive counters, etc. (c) Schools.   (1) A regular speed zone must not change within the limits of a school speed zone since posting of a regular speed zone sign at the point of change would prematurely terminate the school speed zone. This is due to the fact that speed limits remain fixed until a revised limit is encountered.   (2) Speed checks provide a sound basis for selecting the proper speed limits for school zones. While it is not common practice to set speed limits significantly lower than the 85th percentile speed for regulatory speed zones, exceptions to this practice are often found at school zones.   (3) Factual studies, reason, and sound engineering judgment should govern the final decision on the maximum deviation from the 85th percentile speed which will provide a reasonable and prudent speed limit.   (4) It is not advisable to set a school speed limit above 35 miles per hour in either rural or urban areas. Lower school speed limits should be considered when the 85th percentile speed is below 50 miles per hour.   (5) When the results of a speed study indicate an 85th percentile speed at or below 50 miles per hour, the reduced school speed limit should not be more than 15 miles per hour below the 85th percentile speed or normal posted speed limits. If the 85th percentile speed is 55 miles per hour, the reduced school speed limit should be 20 miles per hour below the 85th percentile speed. Any roadway with an 85th percentile speed greater than 55 miles per hour requires a buffer zone to transition down to a 35 mile per hour speed limit.   (6) Operating School Buffer Zones With School Zones.     (A) Establishing buffer zones. In some cases, it may be appropriate to operate the buffer zone during the same time periods that the school speed zone operates. This will allow motorists to travel at the higher posted speeds through both zones when the slower speeds are not necessary. An example of this would be highway with a regular posted speed limit of 70 mph and a posted school zone speed limit of 35 mph. It would be appropriate to have a school transition speed zone of 55 mph that flashes with the 35 mph school zone on either side. This design makes for better public relations because people are not encouraged to violate or disrespect the law when driving through permanently fixed transition zones that 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. Cont'd...