2018-044 Speed Limit StudiesDate: April 13, 2018 Report No. 2018-044
INFORMAL STAFF REPORT
TO MAYOR AND CITY COUNCIL
SUBJECT:
Speed limits along roadways in the City of Denton.
EXECUTIVE SUMMARY:
During concluding items of the March 20, 2018 City Council meeting, Council Member
Hudspeth inquired if a city-wide speed limit analysis could be performed. City-wide speed limit
studies are recommended to be conducted every 5-7 years; staff is expecting to propose a city-
wide speed limit study in the next 18-24 months after construction on major thoroughfares and
arterials is completed.
Speed limits are established based on engineering studies that include speed surveys to determine
the 85th percentile speed limit. The 85th percentile speed is the speed at which 85%of the drivers
drive safely and comfortably and is the basis to determine speed limits. The posted speed limits
on all City and Texas Department of Transportation (TxDOT) streets is determined using a
process that is followed by other agencies throughout the nation and is in accordance with the
state and Federal guidelines. Currently the City has approximately 1,350 lane miles of streets and
TxDOT has 240 miles of streets within the City limits.
BACKGROUND:
A speed limit study is initiated when there are capacity enhancements along a thoroughfare (i.e.
street widening). For example, when State Highway 380 was widened recently, TxDOT
conducted a speed limit study and it was determined that the speeds be increased to 40 MPH.
When the Mayhill and Bonnie Brae projects are complete, the City will initiate a speed study
through these corridors to determine the appropriate speed limits.
A citywide speed limit study of major arterials and thoroughfares is recommended every 5-7
years to determine the appropriate speeds as conditions change within a community. The cost of
the speed study is estimated to cost $70,000-$80,000. A Citywide speed study has not recently
been conducted in Denton due to the constant roadway construction activity in the community.
The roadway construction affects the driver behavior due to detours, lane closures, and other
factors which does not provide a true representation of the appropriate speed of a road. Staff
recommends proposing a city-wide speed limit study in the next 18-24 months after construction
on major thoroughfares and arterials is completed.
Speed limits along City and State roadways are typically established based on engineering
studies. The primary criteria used in establishing a speed limit is called the 85th percentile. In the
case of local residential streets, the prima facie speed limit (not posted) is consistent with state
minimum speed limit of 30 miles per hour in accordance with the Texas Transportation Code.
Use of the 85th percentile speed concept is based on the theory that:
the large majority of drivers:
o are reasonable and prudent
o do not want to have a crash
Date: April 13, 2018 Report No. 2018-044
o desire to reach their destination in the shortest possible time
a speed at or below which 85% of people drive at any given location under good weather
and visibility conditions may be considered as the maximum safe speed for that location.
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% and 85% of the total number of observations (see Figure). 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. 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.
The example illustrated in adjoining figure shows that a speed posted for 7 MPH below the 85th
percentile speed would unfairly penalize 25% of the
drivers 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 MPH of the 15th percentile value. Experience
proves these findings valid and shows that the 85th
percentile speed is a characteristic of traffic speeds that
most closely conforms to a safe and reasonable speed
limit.
As discussed earlier, speed limits are established by
conducting speed surveys calculated at the 85th percentile.
The speed limit is usually rounded to the 5 MPH
increment that is just higher or lower than the 85% (unless
the 85% is calculated at a 5MPH increment). Other
factors that can be considered in lowering a speed limit
less than the 5MPH increment are:
roadway pavement width of 20ft or less,
horizontal and vertical curves,
hidden driveways and other roadside developments,
high driveway density,
a high crash history,
substantial pedestrian activity,
rural residential or development area and,
lack of striped and improved shoulders.
Date: April 13, 2018 Report No. 2018-044
The State of Texas considers these other factors as being a means of establishing a speed limit up
to 10MPH (12MPH for locations with a crash rate higher than the statewide average) lower than
the 85th percentile speed.
STAFF CONTACT:
Pritam Deshmukh
Traffic Engineer
(940) 349-7710
Pritam.Deshmukh@cityofdenton.com