(The material in this section is adapted from Policy on Establishing and Posting Speed Limits on the State Highway System, published by the Illinois Department of Transportation (March 2011).)

Illinois statutes and the State Manual on Uniform Traffic Control Devices require that speed limits other than statutory speed limits be based on "... an engineering study that has been performed in accordance with traffic engineering practices. The engineering study shall include an analysis of the current speed distribution of free-flowing vehicles."

The following procedure shall be used to determine speed zones on streets and highways under the jurisdiction of the DOT. The same procedure is recommended for local agencies.

STEP 1: Establish the Prevailing Speed

The prevailing speed is the average of the following three metrics, measured during free-flowing traffic conditions:

• 85th Percentile Speed: The speed at or below which 85 percent of the vehicles are traveling.
• Upper Limit of the 10 mph Pace: The 10 mph range containing the most vehicles.
• Average Test Run Speed: Determined on the basis of five vehicle runs in each direction over the length of the proposed speed zone.

The prevailing speed is the nearest 5 mph increment to the average of the above three values.

STEP 2: Supplementary Investigations (Optional)

Adjustment factors for determining the proposed posted speed limit may be determined by further investigation of any or all of the following four conditions:

• Elevated Crash Risk: If the speed zone being studied contains a portion of a high-crash segment or contains a high-crash intersection as determined by the Bureau of Safety Engineering, the prevailing speed may be reduced by 10 percent.
• Access Control: The access conflict number (ACN) is calculated for the speed zone, and this number is used to determine the percent reduction of the prevailing speed as shown below.

(Extended Text Description: Equation: *ACN = (N_s+5N_m+10N_i) / L)

Where:

N_s = Number of field entrances and driveways to single-family dwellings

N_m = Number of driveways to minor commercial entrances, multi-family residential units, and minor street intersections

N_i = Number of driveways to major commercial entrances, large multi-family developments, and major street intersections

• Pedestrian Activity: Where no sidewalks are provided or where sidewalks are located immediately behind the curb and the total pedestrian traffic exceeds 10 per hour for any 3 hours within any 8-hour period, the prevailing speed may be reduced by 5 percent. Pedestrians crossing the route at intersections or established crossing points may be included if the point of crossing is not controlled by a STOP or YIELD sign on the route in question, or does not have traffic signals.
• Parking: The prevailing speed may be reduced by 5 percent where parking is permitted adjacent to the traffic lanes.

The adjustment factors from the four different factors are added together to produce a single percentage adjustment that shall not exceed 20 percent.

Step 3: Selection of Preliminary Speed Limit

The preliminary speed limit is either the calculated prevailing speed (from Step 1), or if the optional investigation was undertaken, it is the prevailing speed as adjusted by application of the percentage corrections from the optional investigation (Step 2). The following rules apply to the outcome:

• The preliminary posted speed limit should be the closest 5 mph increment to the (adjusted) prevailing speed.
• The preliminary posted speed limit shall not differ from the prevailing speed (from Step 1) by more than 9 mph or by more than 20 percent, whichever is less.

Step 4: Violation Check

Using the spot speed data collected in Step 1, determine the median speed (the 50th percentile). The proposed speed limit should be either the preliminary posted speed limit or the 50th percentile speed, whichever is greater.

If the proposed speed limit exceeds the statutory speed limit for the highway in question, either the statutory speed or the proposed speed limit may be posted. If the selected speed limit results in a violation rate greater than 50 percent, the appropriate police agency(ies) should be notified that extra enforcement efforts may be necessary.

It is noted that differences in posted speeds between adjacent speed zones should not be more than 10 mph. However, the Illinois policy permits a larger difference provided that adequate speed reduction signs are posted.