(A) Basic investigation.
(1) The prevailing speed of free flowing traffic has been recognized by most transportation agencies as the basic parameter for use in establishing speed limits either higher or lower than the statutory speed limits set by 625 ILCS 5/11-601 of the Illinois Vehicle Code.
(2) Prevailing speed shall be determined by use of two separated sets of data, which are:
(a) Eighty-fifth percentile speed;
(b) Upper limit of the consecutive ten mph that have the most observations, also known as the upper limit of the ten mph pace;
(c) The prevailing speed shall be determined by computing the average of the measured values for the criteria listed above; and
(d) Based on the assumption that a majority of motorists will drive at a speed, which they judge to be safe and reasonable, the selected speed limit in five mph increments should not differ from the established prevailing speed by more than three mph unless justified by supplementary investigations.
(B) Optional supplementary investigations.
(1) Selected speed limit. The selected speed limit may differ from the established prevailing speed by more than three mph when justified by further investigation. Such investigation may include the study of any or all the following conditions:
(a) Accident rate;
(b) Access control;
(c) Pedestrian activity; and
(d) Parking.
(2) Accident rate. If the accident rate, based on all reportable accidents (intersection and non-intersection), within the proposed zone is 50% higher than the county average accident rate for the same classification of highway, the prevailing speed may be reduced by 5%. If the accident rate is more than twice the county average rate for the same classification of highway, the prevailing speeds may be reduced by 10%. Minor accidents not reportable under state statutes are not to be considered in computing accident rates.
(3) Access control. The effect of driveways and other entrances will be determined by using an access control number. For this purpose, a field entrance or driveway to a single-family dwelling shall have a conflict number of one.
(a) Minor commercial entrances and driveways serving multiple family residential units and minor street intersections shall have a conflict number of five and major commercial entrances or major street intersections shall have a conflict number of ten.
(b) If the total access conflict number within a proposed zone exceeds the rates shown in the table, the prevailing speed may be reduced by the percentages indicated.
Access Conflict Rate (Conflicts Per Mile) | Prevailing Speed Reduction (Percent) |
0—40 | 0% |
41—60 | 5% |
61 or more | 10% |
(4) Pedestrian activity.
(a) Where no sidewalks are provided or where sidewalks are located immediately behind the curb and the total pedestrian traffic exceeds ten per hour for any three hours within any eight-hour period, the prevailing speed may be reduced by 5%.
(b) Pedestrians crossing the route may be included if the point of crossing is not protected by some other form of speed control or intersection control device. This reduction may be applied only once in the establishment of any particular zone.
(5) Parking. Where parking is permitted adjacent to the traffic lanes, the prevailing speed may be reduced by 5%.
(6) Percentage corrections. Percentage corrections for any or all of the above factors may be applied to the prevailing speed of free-flowing traffic, but in no case shall the resulting speed limit differ from the prevailing speed by more than ten mph or by 20% of the prevailing speed, whichever is less. Posted limits shall always be in five mph increments.
(Res. passed 3-13-2007)