949.2 Speed Limit Guidelines

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

The following guidelines promote consistency of speed limit postings on the state of Missouri’s highway system. In all cases, engineering judgment is necessary to determine the most appropriate speed limit at a particular location.

949.2.1 Maximum Speed Limits

Maximum speed limits in Missouri are governed by the Missouri Revised Statutes, Section 304.010. The following table summarizes maximum speed limits as established by the statutes:

Rural Areas
70 mph Interstate and Freeways
65 mph Expressways
55 mph State Lettered Routes 1
60 mph All other roadways, not urbanized
Urbanized Areas
60 mph Interstate, Freeways and Expressways
55 mph State Lettered Routes 1
1 May be up to 60 mph based on engineering judgement
• At no time shall any Missouri road be posted above 70 mph.
• “…the Highways and Transportation Commission may set a speed limit higher or lower than the uniform maximum speed limit…if a higher or lower speed limit is recommended by the Department of Transportation.”

949.2.1.1 Definitions (per MRS 304.010)

Freeway: A limited access divided highway of at least ten miles in length with four or more lanes that is not part of the federal interstate system of highways and does not have any crossovers or accesses from streets, roads or other highways at the same grade level as such divided highway within such ten miles of divided highway;

Expressway: A divided highway of at least 10 miles long with 4 or more lanes that is not part of the federal interstate system of highways and has crossovers or accesses from streets, roads or other highways at the same grade level as such divided highway;

949.2.1.2 Discrepancies

Before raising a posted speed limit above the maximums in the state statute, the district should notify the State Traffic Engineer.

Districts should coordinate across district borders to ensure consistent posted speed limits on inter-district routes in accordance with the speed limit guidelines.

949.2.1.3 Length of Section

Any speed limit section should have a logical beginning and ending point. Examples include city limits, roadway type changes, etc. Additionally, speed limits in unincorporated or “non-community” areas should stay consistent for a minimum of 2 miles.

949.2.2 Prevailing Speed Determination

Prevailing speed determination is the starting point to setting speed limits in incorporated and unincorporated areas, pursuant to the aforementioned state statutes. The prevailing speed of free flowing traffic shall be determined using one or more of the three criteria below:

• 85th percentile speed;
• Upper limit of the 10 mph pace;
• Average test run speed.

The selected speed limit (in 5 mph increments) should not exceed the established prevailing speed by more than 3 mph.

The following factors may allow for reduction of the prevailing speed. The prevailing speed shall not be reduced below the 50th percentile (average) speed using these factors.

1. Fatality and Disabling Injury Crash Rate
2. Total Crash Rate
3. Pedestrian Traffic
4. Parking

942.2.2.1 Fatality and Disabling Injury Crash Rate

If either the fatal or disabling injury crash rates, based on reportable accidents within the proposed area, are 50% higher than the statewide average crash rate for the same roadway type, prevailing speed may be reduced by 5%.

If the fatal or disabling crash rate is more than twice the statewide rate for the roadway type, the prevailing speed may be reduced by 10%.

Table 949.2.2.1 Prevailing Speed Reduction for Severe Crash Rates

Reduction Condition Prevailing Speed Reduction
Fatal or Disabling Injury Crash Rate > 1.5 SWAR 5%
Fatal or Disabling Injury Crash Rate > 2.0 SWAR 10%

942.2.2.2 Total Crash Rate

If the overall crash rate, based on reportable accidents within the proposed area, is 50% higher than the statewide average crash rate for the same roadway type, prevailing speed may be reduced by 5%.

If the crash rate is more than twice the statewide rate for the roadway type, the prevailing speed may be reduced by 10%.

Table 949.2.2.2 Prevailing Speed Reduction for Total Crash Rates

Reduction Condition Prevailing Speed Reduction
Total Crash Rate > 1.5 SWAR 5%
Total Crash Rate > 2.0 SWAR 10%

949.2.2.3 Pedestrian Traffic

Where sidewalks are not provided adjacent to the route and a total pedestrian traffic along the route exceeds 10 per hour for 3 hours of any 8-hour period, the prevailing speed may be reduced by 5 percent. Pedestrians crossing the route may only be counted if the point of crossing is not protected by a traffic control device.

949.2.2.4 Parking

Where parking is permitted adjacent to the traffic lane, the prevailing speed may be reduced by 5 percent.

Where significant adjacent development includes residential and commercial access points, the prevailing speed may be reduced to account for driveway conflicts. If necessary, this affect on the prevailing speed may be determined as outlined below.

The effect of driveway entrances can be determined by using a Driveway Conflict Number. For this purpose, a private or field entrance should have a Driveway Conflict Number of 1. Minor commercial entrances should have a number of 5. Major commercial entrances, shopping centers or industrial plant entrances that generate relatively high volumes of traffic, and public streets should have a number of 10. If the total Driveway Conflict Number exceeds a rate of 40 per mile through the proposed zone, the prevailing speed may be reduced by 5 percent. If the total number exceeds a rate of 60 per mile, the prevailing speed may be reduced by 10 percent. However, before a reduction is made due to the Driveway Conflict Number, the Poisson Curve should be used to determine if the accident reduction is statistically significant.

Figure 949.2.2.5

The test is as follows:

a) Consult Fig. 949.2.2.5, using the current number of accidents for the last year.

b) Determine the graph’s percent value from the Poisson Curve.

c) Compute accident rate for last year.

${\displaystyle {\mbox{Accident Rate}}={\frac {\mbox{no. of crashes x 100,000,000}}{\mbox{365 x ADT x length in miles}}}}$

d) Obtain Statewide Accident Rate (SWAR). The accident data is obtained from the Transportation Management System (TMS).

e) Calculate percent reduction between accident rate and statewide accident rate by:

${\displaystyle {\mbox{Percent Reduction}}={100}{\frac {\mbox{AR-SWAR}}{\mbox{AR}}}}$

f) Compare % reduction with value obtained in step b.

g) If % reduction equals or exceeds value obtained in step b, the Driveway Conflict Number may be used as the action would be statistically significant.

Example:

8.0-mile section of roadway, ADT = 3,100, 30 accidents within last year.
a & b). Value obtained from Fig. 949.2.2 = 30 %
c).${\displaystyle {\mbox{AR}}={\mbox{331.4}}={\frac {\mbox{30(100,000,000)}}{\mbox{(365)(3,100)(8.0)}}}}$
d). SWAR = 242.04
e).${\displaystyle {\mbox{26.9 percent reduction}}={100}{\frac {\mbox{331.4-242.04}}{\mbox{331.4}}}}$
f). Compare % reduction with value.
26.9 < 30
Since 26.9 is less than 30, the Driveway Conflict Number cannot be considered.

949.2.2.6 Procedures for Obtaining Prevailing Speed Data

Spot speed studies can be made with any speed measurement device such as radar, count cards and inductive loops that determine the 85th percentile speed and the upper limit of the 10 mph pace. The 85th percentile speed is defined as the speed at or below which 85 percent of the vehicles are traveling. The 10 mph pace is defined as the 10 mph range containing the most vehicles.

Spot speed studies should be made as close to the center of the proposed zone as is practical. If the zone is lengthier than one mile, studies should be made at a minimum of two locations. If the difference in data between the two locations is minor, the higher value should be used. If the difference in data is over 5 mph, consideration should be given to designating separate zones.

Exercise care to collect the data in a manner and at times that indicate normal conditions. Normal conditions will be assumed to prevail under good weather conditions on dry pavement, following morning rush hours and prior to the evening rush hours, on any day of the week except Saturdays, Sundays or holidays. Observations should not be made immediately following a significant event or during a period of greater than normal police enforcement. Every effort should be made to disguise or conceal the fact that speeds are being reported.

Speeds should be observed for at least 100 passenger cars and pick-up trucks. Trucks over 4 tons will not be included in the data for determining a revised limit.

Average test run speeds shall be determined on the basis of at least two runs in each direction over the length of the proposed zone. The prime consideration in these test runs is to determine a maximum permissible speed. Speeds are to be recorded at 0.1 mile intervals. While making the test run, the driver will try to “float” in the traffic stream, passing as many vehicles as pass the test car.

949.2.3 Additional Guidelines for Establishing Speed Limits

Normally, isolated curves and turns, areas of restricted sight distance, no passing zones, etc. will not be considered as areas for lowering speed limits. The accident record on such highways will be the principle criterion. Critical speed signs shall be used in connection with the appropriate warning signs at these locations.

Except on divided highways, different speeds for traffic in opposite directions cannot be justified.

Speed limits shall begin at a point on or as near in advanced of the point as possible where the speed limit is warranted and shall end at the point or as near the point as possible where the speed limit is not warranted.