1. (a) Overview.

   1. (1) Engineering and traffic investigation.

      1. (A) This section describes 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. (B) Sound and generally accepted engineering practices are an integral part of speed zone studies and are discussed in subsection (d) of this section.

   2. (2) Scope of study.

      1. (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.

      2. (B) A speed zone study consists of the following principle areas:

         1. (i) determining the 85th percentile speed;
         2. (ii) crash study;
         3. (iii) developing of strip maps;
         4. (iv) speed zone design; and
         5. (v) rechecks of speed zones.

2. (b) Determining the 85th percentile speed.

   1. (1) General concepts.

      1. (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.
      2. (B) The 85th percentile speed is a value that is used by many states and cities for establishing regulatory speed zones.

   2. (2) Theory. Use of the 85th percentile speed concept is based on the theory that:

      1. (A) a speed at or below which 85 percent of people drive at any given location under good weather and visibility conditions may be considered as the maximum safe speed for that location; and

      2. (B) the large majority of drivers:

         1. (i) are reasonable and prudent;
         2. (ii) do not want to have a crash; and
         3. (iii) desire to reach their destination in the shortest possible time.

   3. (3) Statistical rationale.

      1. (A) The results of numerous and extensive before-and-after studies substantiates the general propriety and value of the 85th percentile criterion.
      2. (B) Statistical techniques show that a normal probability distribution will occur when a random sample of traffic is measured. From the resulting frequency distribution curves, one finds that a certain percentage of drivers drive too fast for the existing conditions and a certain percentage of drivers travel at an unreasonably slow speed compared to the trend of traffic.

      3. (C) Most cumulative speed distribution curves break at approximately 15% and 85% of the total number of observations as shown in the following Figure 1.
         Attached Graphic
         

         1. (i) Consequently, the motorists observed in the lower 15% are considered to be traveling unreasonably slow; and those observed above the 85th percentile value are assumed to be exceeding a safe and reasonable speed.
         2. (ii) Because of the steep slope of the distribution curve below the 85th percentile value, it can readily be seen that posting a speed below the critical value would penalize a large percentage of reasonable drivers.

      4. (D) The example illustrated in Figure 1 as shown in subparagraph (C) of this paragraph shows that a speed posted for 7 miles per hour below the 85th percentile speed would unfairly penalize 25% of the drivers who would otherwise be considered to be driving at a reasonable and prudent speed. Therefore, for purposes of speed zoning:

         1. (i) the maximum posted speed should be as near as possible to the 85th percentile value, and
         2. (ii) whenever minimum speed zones are used, the minimum posted speed should be within 5 miles per hour of the 15th percentile value (See 25.22(b) of this title (relating to Regulatory Speed Zones) for additional information on minimum speed limits).
      5. (E) Experience proves these findings valid and shows that the 85th percentile speed is the one characteristic of traffic speeds that most closely conforms to a speed limit which is considered safe and reasonable.

   4. (4) Speed checks for existing highways are of prime importance, because they:

      1. (A) represent the consensus of drivers as to the safe speed at a given location; and
      2. (B) provide the basic data on which the regulatory speed zone is based.

   5. (5) Speed checks for new or reconstructed highways.

      1. (A) Speed checks on new or reconstructed highway sections should not be performed until it is apparent that the traffic speeds have stabilized.
      2. (B) As an interim measure, the statewide maximum speed or the design speed of the roadway may be posted on these sections while utilizing warning signs with advisory speed signs to alert drivers to any hazards. In any case, trial run data should be collected and considered by a traffic engineer before interim speed zones are posted. Once the traffic speeds have stabilized, normal speed zone studies should be completed and evaluated by a traffic engineer before the final speeds are posted.
      3. (C) 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.

   6. (6) Operation of speed check stations.

      1. (A) Normal speed checks should:

         1. (i) be made on average week days during off-peak hours under normal traffic conditions;
         2. (ii) be made under favorable weather conditions;
         3. (iii) include only "free floating" vehicles (see subparagraph (B) of this paragraph);
         4. (iv) include a minimum of 125 cars in each direction at each station; and
         5. (v) be discontinued after two hours, even if 125 cars have not been timed.

      2. (B) The vehicles checked should be only those in which drivers are choosing their own speed ("free floating").

         1. (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.
         2. (ii) Cars involved in passing or turning maneuvers should not be checked, because they are probably driving at an abnormal rate of speed.
      3. (C) Trucks and buses should be recorded separately and should not be included as part of the 125-car total.

   7. (7) Location of speed check stations.

      1. (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.

      2. (B) In urban areas and on approaches to cities, speed check stations:

         1. (i) should generally be located at intervals of 0.40 kilometer (0.25 mile) or less if necessary to insure an accurate picture of the speed pattern;
         2. (ii) should be located midway between signals or 0.3 kilometer (0.2 miles) from any signal, whichever is less, to ensure an accurate representation of speed patterns;
         3. (iii) should take into account the locality and the uniformity of physical and traffic conditions;
         4. (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
         5. (v) should be checked midway between interchanges on the main lanes of expressways and freeways.

      3. (C) In rural areas, speed check stations:

         1. (i) may be at intervals greater than 0.40 kilometer (0.25 mile), as long as the general speed pattern is followed;
         2. (ii) may only be necessary at each end and the middle point in low volume sections; and
         3. (iii) may be determined by trial runs through the area.

   8. (8) Measuring speeds.

      1. (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.
      2. (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.

   9. (9) Recording measured speeds.

      1. (A) Use TxDOT Form 1882, "Radar Motor Vehicle Speed Field Tally Sheet," to record tally marks beside the observed speed for each vehicle. Copies may be obtained from the Traffic Operations Division.
      2. (B) The following Figure 2 shows an example of a completed Field Tally Sheet.
         Attached Graphic
         

   10. (10) Calculating 85th percentile speed.

       1. (A) Use the following procedure to calculate the 85th percentile speed.

          1. (i) Add the tally marks as shown in the Cumulative Total column in Figure 2 as shown in paragraph (9)(B) of this subsection. Note that the marks are added from the bottom up.
          2. (ii) For each Cumulative Total column, calculate 85% of the total number of vehicles checked. (EXAMPLE: Figure 2 shows that 125 cars were counted in the northbound direction. So 85% would be 106 (125 x 0.85 = 106). Thus, the 106th car (counting up from the bottom) represents the 85th percentile speed.)
          3. (iii) Determine the speed at which the car representing the 85th percentile was traveling. Again from the northbound example in Figure 2, the 106th car was traveling at 48 miles per hour. Thus, 48 miles per hour is the 85th percentile speed.
       2. (B) In Figure 2 as shown in paragraph (9)(B) of this subsection, there is an "85th Percentile Calculation Table," which may be used to readily determine the 85th percentile car if the total number is between 80 and 239.
       3. (C) In no case shall the 85th percentile speed be interpolated between two speeds in the M.P.H. column.
       4. (D) After the locations of the speed check stations have been decided upon and the speed checks have been made, the 85th percentile speeds should be calculated immediately in the field. By doing so, it is possible to get an idea of what the speed curve will look like and to determine if more speed check stations are needed.

   11. (11) Recording the information.

       1. (A) Record the speed check data on the strip map as described in subsection (c) of this section.
       2. (B) The blocks shown on the strip map contain the 85th percentile speed, the speed of the fastest car checked, and the number of cars checked (reading from top to bottom in order).
       3. (C) Show a block for each speed check location for each direction of travel measured.
   12. (12) Incomplete data. When the data appear incomplete because of a large change in the 85th percentile speed between speed check stations or an unusually high or low 85th percentile speed at a particular point, additional speed checks should be made at new locations or repeat checks should be made at certain locations previously checked to clarify the speed picture.

3. (c) Developing strip maps

   1. (1) Strip map blanks.

      1. (A) The first step in establishing a speed zone is the preparation of the strip map on which the data pertinent to the speed zone are recorded.
      2. (B) The map should be drawn on a strip map blank. These blanks are available at the Traffic Operations Division as a graphics file or as printed blanks. The following Figure 3 shows a sample strip map blank reduced in size.
         Attached Graphic
         
      3. (C) The blank is intended to ensure uniformity and reduce drafting work in the districts.

   2. (2) Scale for strip map.

      1. (A) The scale used on the Road Inventory (RI-1) Highway Log Sheets is generally suitable for the strip map. In some cases, however, such as in highly developed areas, a scale of 1 inch = 300 feet is preferred. In sparsely developed rural areas, the RI-1 scale should be satisfactory. See the following Figure 4 for an example RI-1 sheet.
         Attached Graphic
         
      2. (B) Two different scales should not be used on the same strip map sheet. If, in zoning a long continuous section of highway, it is desirable to change the scale on the strip map, then another strip map sheet should be used for the beginning of the new scale.

   3. (3) Field entry data.

      1. (A) The strip map should be made complete by entering field data to show road details in every aspect. The headings on the strip map blank indicate much of the required data. The following Figure 5 through Figure 8 show examples of how the data should be presented.
         Attached Graphic
         
      2. (B) If the roadway is not divided, it is not necessary to repeat the right of way and roadbed width, surface width and type, grades over three percent, and curves over two degrees for both directions of travel.
      3. (C) In addition to the preprinted information on the far left of the strip map blank, the information in the following table should be shown:
         Attached Graphic
         

   4. (4) City limits.

      1. (A) At locations where the study is extended into or through an incorporated city, the zone should be broken at the city limit, regardless of whether the zone speed changes. This is necessary because the speed limits inside the limits of an incorporated city may be established by the city government, while the speed limits outside the corporate limits of a city must be established by the commission.
      2. (B) It is also necessary to show on the strip map the precise location of the city limit either by reference marker or milepoint and control section to prevent misinterpretation of the zone when the city limit is changed. The zone speed of any adjoining map should always be shown.
      3. (C) In the event the city limit is changed, the strip map and commission minute order should be revised to show the new city limit.

   5. (5) Schools.

      1. (A) If a reduced school speed limit is warranted, a speed zone strip map should be prepared as shown in the following Figures 10 and 11.
         Attached Graphic
         
      2. (B) 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.
      3. (C) 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.
      4. (D) Factual studies, reason, and sound engineering judgement, rather than emotion, should govern the final decision on the maximum deviation from the 85th percentile speed which will provide a reasonable and prudent speed limit.
      5. (E) 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.
      6. (F) When the results of a speed study indicate an 85th percentile speed 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. (6) Showing crashes on strip maps.

      1. (A) At locations where recommended speed zones will be 5 miles per hour or more below the 85th percentile speed because of high crash experience, the crashes for the most recent calendar year available should be shown on the strip map. RI-1 Sheets will facilitate accurate plotting of crashes, since crash locations are coded from these sheets.
      2. (B) When districts submit strip maps or prints to the Traffic Operations Division for review, TRF will obtain the crash rate for the roadway section in question as well as the statewide average crash rate for the appropriate type of roadway section and add these data to the strip map. Crash rates will be considered before lowering the zone. These data will be an important consideration in determining whether the lower zoning is justified.
      3. (C) Crash data need not be plotted on the strip map when proposed speed zones are within 5 miles per hour of the 85th percentile speed checks.

4. (d) Speed zone design.

   1. (1) Zone length.

      1. (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 0.322 kilometer (0.200 mile) or more.

      2. (B) School zones are the exception to this rule and may be as short as reasonable in urban areas, depending on approach speeds.

         1. (i) School zones in urban areas where speeds are 30 miles per hour or less may have school zones as short as 60.96 to 91.44 meters (200 to 300 feet).
         2. (ii) Where speeds exceed 40 miles per hour, the minimum school zone length should be 304.80 meters (1,000 feet) to allow for normal deceleration.

   2. (2) Transitions.

      1. (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.
      2. (B) If adjacent 85th percentile speeds show an abrupt change of more than 15 miles per hour, a transition zone of approximately 0.32 kilometer (0.200 mile) or more in length should be used.
   3. (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. (4) Directional differences.

      1. (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.
      2. (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.
      3. (C) On undivided roadways, the zones in opposite directions should be the same for enforcement purposes.

   5. (5) Variation from 85th percentile.

      1. (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 determined by the study, based on the professional judgement of the supervising engineer. Only under special conditions would the zone speed vary further from the 85th percentile. Explanations of such conditions follow.

         1. (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.
         2. (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 7 miles per hour lower than the 85th percentile speed. NOTE: 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.
         3. (iii) Light traffic volumes. At locations where traffic volumes are light and 125 cars cannot be checked in the two 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 in paragraph (7) of this subsection.) Trial runs may be documented using the "Summary of Trial Run for Speed Zones" (TxDOT Form 1929) instead of a strip map. Copies may be obtained from the Traffic Operations Division. The following Figure 12 shows an example of a completed Summary of Trial Run for Speed Zones.
            Attached Graphic
            
         4. (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. Trial runs may be documented using the Summary of Trial Run for Speed Zones (TxDOT Form 1929) instead of a strip map. Speed zones established through this process should be rechecked in accordance with the procedure in subsection (e) of this section.

         5. (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, based on sound and generally accepted engineering judgement that includes consideration of the following factors:

            1. (I) narrow roadway pavement widths: 20 feet (6.1 m) or less, for example;
            2. (II) horizontal and vertical curves (possible limited sight distance);
            3. (III) hidden driveways and other developments (possible limited sight distance);
            4. (IV) high driveway density (the higher the number of driveways, the higher the potential for encountering entering and turning vehicles);
            5. (V) crash history along the location;
            6. (VI) rural residential or developed areas (higher potential for pedestrian and bicycle traffic); and
            7. (VII) lack of striped, improved shoulders (constricted lateral movement).
      2. (B) 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)).
      3. (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.
      4. (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, 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. (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. (7) Trial runs.

      1. (A) 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. An average passenger vehicle along with a reasonably competent driver should be selected. An "average passenger vehicle" should be somewhat representative of most vehicles on the highway.
      2. (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.
      3. (C) Documentation. Trial runs may be documented using the Summary of Trial Run for Speed Zones (TxDOT Form 1929) instead of a strip map.

   8. (8) Location of regulatory speed limit signs.

      1. (A) Speed zones are legally described to the nearest thousandth of a mile (5 feet). Regulatory speed limit signs should be located within approximately 1.6 meters (5 feet) of the actual reference marker or milepoint defined in the minute order or city ordinance or resolution.
      2. (B) The locations of regulatory speed zones tied to speed changes should be examined carefully to ensure that signs can be erected within the 1.6 meters (5 feet) variation. If adherence to the 1.6 meters (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.

5. (e) Rechecks of speed zones.

   1. (1) Introduction.

      1. (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.
      2. (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. (2) Frequency of rechecks.

      1. (A) Periodic rechecks of all zones are desirable at intervals of about three to five years in urban areas regardless of roadway improvements, roadside developments, or increases in traffic volumes. Trial runs or rechecks of every third speed check station may be made.
      2. (B) Rechecks in rural areas are desirable at intervals of five to ten years. In many instances, trial runs may be sufficient.
      3. (C) If the speed checks or trial runs indicate a need for revision of the zone, rechecks of speeds should be made at all speed check stations for that particular section and a revised strip map made and submitted.

   3. (3) Procedure.

      1. (A) In preparing a revised strip map, the original tracing on file with the Traffic Operations Division may be obtained and used where all data, other than speed check data shown on the map, are current. A strip map containing out of date information should be considered obsolete.
      2. (B) New strip maps should be prepared when physical improvements of the roadway have been made or when changes in roadside development have occurred since the original strip map was prepared even though the speed checks and zone speeds may not change.

Source Note:The provisions of this §25.23 adopted to be effective February 17, 2000, 25 TexReg 1151.