85th Percentile and speed.
General Concepts
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. The 85th percentile speed is a value that is used by many states and
cities for establishing regulatory speed zones.
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.
Theory
Use of the 85th percentile speed concept is based on the theory
that:
- the large majority of drivers:
- are reasonable and prudent
- do not want to have a crash
- desire to reach their destination in the shortest possible time
Statistical Rationale
The results of numerous and extensive “before-and-after” studies
substantiates the general propriety and value of the 85th percentile criterion.
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.
Most cumulative speed distribution curves “break” at approximately
15 percent and 85 percent of the total number of observations (see Figure 3-1).
Consequently, the motorists observed in the lower 15 percent 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. 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.
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Figure 3-1. Cumulative speed distribution curve
The example illustrated in Figure 3-1 shows that a speed posted
for 7 miles per hour below the 85th percentile speed would unfairly penalize 25
percent of the drivers who would otherwise be considered to be driving at a
reasonable and prudent speed. Therefore, for purposes of speed zoning, the
maximum posted speed should be as near as possible to the 85th percentile value,
and whenever minimum speed zones are used, the minimum posted speed should be
within 5 miles per hour of the 15th percentile value. (See Chapter 2, Section 2,
for additional information on Minimum Speed Limits. )
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.
Speed Checks for Existing Highways
Speed checks are of prime importance, because they:
- represent the consensus of drivers as to the safe speed at a given location
- provide the basic data on which the regulatory speed zone is based.
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.
Operation of Speed Check Stations
Normal speed checks should:
- be made on average week days at off-peak hours
- be made under favorable weather conditions
- include only “free floating” vehicles (see following paragraph)
- include a minimum of 125 cars in each direction at each station
- be discontinued after two hours if radar is used, or after four hours if a traffic counter that classifies vehicles by type is used — even if 125 cars have not been timed.
The vehicles checked should be only those in which drivers are
choosing their own speed (“free floating”). 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. Cars involved in passing or turning maneuvers should not be
checked, because they are probably driving at an abnormal rate of speed.
Trucks and busses should be recorded separately and should not be
included as part of the 125-car total.
Location of Speed Check Stations
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.
In urban areas and on approaches to
cities, speed check stations:
- should generally be located at intervals of 0.25 mile or less if necessary to insure an accurate picture of the speed pattern
- should be located midway between signals or 0.2 miles from any signal, whichever is less, to ensure an accurate representation of speed patterns
- should take into account the locality and the uniformity of physical and traffic conditions
- 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
- should be checked midway between interchanges on the main lanes of expressways and freeways.
In rural areas, speed check stations:
- may be at intervals greater than 0.25 mile, as long as the general speed pattern is followed
- may only be necessary at each end and the middle point if the characteristics of the roadway are consistent throughout the entire section
- 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 within the two hours if radar is used, or after four hours if a traffic counter that classifies vehicles by type is used.
Measuring Speeds
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.
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.
Recording Measured Speeds
Use TxDOT Form 1882, “Radar Motor Vehicle Speed Field Tally Sheet,” to record
tally marks beside the observed speed for each vehicle. The form is available
via hyperlink (click on the form number above) or from the Traffic Operations
Division.
Forms for high speed (Form 1882HS) and low speed (Form 1882LS) roads are also available via hyperlink from the
Traffic Operations Division.
Figure 3-2 shows an example of a completed Field Tally
Sheet.
Calculating 85th Percentile Speed
Use the following procedure to calculate the 85th percentile
speed:
- Add the tally marks as shown in the “Cumulative Total” column in Figure 3-2. Note that the marks are added from the bottom up.
- For each “Cumulative Total” column, calculate 85 percent of
the total number of vehicles checked.
EXAMPLE: Figure 3-2 shows that 125 cars were counted in the northbound direction. So 85 percent would be 106 (125 x 0.85 = 106). Thus, the 106th car (counting up from the bottom) represents the 85th percentile speed.
- Determine the speed at which the car representing the 85th percentile was traveling. Again from the northbound example in Figure 3-2, the 106th car was traveling at 48 miles per hour. Thus, 48 miles per hour is the 85th percentile speed.
