lasalle county speed limit policy
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Establishment of Speed Limits Policy


A. When the County Board of LaSalle County determines that a maximum speed limit is greater or less than is reasonable or safe with respect to the conditions found to exist along any county or township highway, or part or zone thereof, within LaSalle County, except those that are under the jurisdiction of the Illinois Department of Transportation or the Illinois State Toll Highway Authority; the County Board shall determine and declare by ordinance a reasonable and proper absolute maximum speed limit as follows:

  1. Does not exceed 55 mph.
  2. Increases the limit within an urban district to not more than 55 mph.
  3. Decrease the limit within an urban district to not less than 20 mph.
  4. Decreases the limit outside of an urban district to not less than 35 mph, except as otherwise provided in subparagraph 5 of this paragraph.
  5. Decreases the limit within a residence district to not less than 25 mph, except as otherwise provided in subparagraph 3 of this paragraph.
  6. Not more than six alterations shall be made per mile, and the difference in limit between adjacent altered speed zones shall not be more than 10 miles per hour.
  7. Minimum speed limits may be posted at any locations where it has been determined that the slow vehicles are impeding the flow of traffic.



A. 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 Section 11-601 of the Illinois Vehicle Code.

B. Prevailing speed shall be determined by use of two separate sets of data, which are:

  1. Eighty-fifth percentile speed.
  2. Upper limit of the consecutive 10 miles per hour that have the most observations, also known as the upper limit of the 10 mph pace.

C. The prevailing speed shall be determined by computing the average of the measured values for the criteria listed above.

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 5 miles per hour increments should not differ from the established prevailing speed by more than 3 mph unless justified by supplementary investigations.


A. The selected speed limit may differ from the established prevailing speed by more than 3 miles per hour when justified by further investigation. Such investigation may include the study of any or all the following conditions:

  1. Accident Rate.
  2. Access Control.
  3. Pedestrian Activity.
  4. Parking.

B. Accident Rate
If the accident rate, based on all reportable accidents (intersection and nonintersection), within the proposed zone is 50 percent higher than the Statewide average accident rate for the same classifications of highway, the prevailing speed may be reduced by 5 percent. If the accident rate is more than twice the Statewide average for the same classification of highway, the prevailing speed may be reduced by 10 percent. Minor accidents not reportable under State statutes are not to be considered in computing accident rates. Current information concerning accident rates will be maintained by and is available from the Illinois Department of Transportation.

C. Access Control
The effect of driveways and other entrances will be determined by using an access conflict number. For this purpose, a field entrance or driveway to single-family dwelling shall have a conflict number of one. 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. 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 percentage indicated.

Access Conflict Rate Prevailing Speed Reduction (Conflicts Per Mile) (Percent)

  • 0-40 0
  • 41-60 5
  • 61 or more 10

D. Pedestrian Activity
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 threehours within any eight-hour period, the prevailing speed may be reduced by 5 percent. 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

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

F. 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 10 miles per hour or by 20 percent of the prevailing speed, whichever is less. Posted limits shall always be in 5 miles
per hour increments.


A. Spot speed studies are to be analyzed to determine the 85th percentile speed and the upper limit of the 10 mile per hour pace. The 85th percentile speed is defined as the speed at which 85 percent of the vehicles are travelling at or below. The 10 miles per hour pace is defined as the 10 miles per hour range containing the most vehicles.

B. Spot speed studies should be made as close to the center of the proposed zone as is practicable. If the zone is in excess of one mile in length studies should be made every mile. If the difference in prevailing speed between the two adjacent locations is less than 5 miles per hour, the lower value should be used. If the difference in prevailing speed is over 5 miles per hour, consideration should be given to designating two separate zones.

C. Care must be exercised to be sure that the data is collected in such a manner and at such times that it is a true indication of normal conditions. Such conditions are assumed to prevail under clear weather conditions, dry pavement, following the morning peak traffic hours and prior to the evening peak traffic hours on any weekday except holidays. The traffic study shall not be taken the day prior to a holiday or the day after a holiday. Observations should not be made immediately following a spectacular accident or during a period of greater than normal police enforcement. Every effort should be made to conceal the fact that speeds are being recorded.

D. Speeds should be observed for a total of 100 passenger cars and pickup trucks in each direction (a maximum of 3 hours of observation can be imposed on low volume highways). Speed of trucks over 4 tons shall not be included in the data for determining an altered limit.

E. On rural residential subdivision streets, where access to said streets exceeds 50 points per mile for a distance of not less than 1000 feet, the recommendation by the County Engineer to the County Board for an altered speed limit may be based on the application of engineering judgment, using an abbreviated traffic investigation, without establishing a prevailing speed. Reductions shall be limited to the design speed of the roadway, but not less than 25 mph. This exception is based on the premise that unposted urban streets have a maximum statutory speed limit of 30 mph. Similarly, a determination of the prevailing speed should not be required for every rural residential street. This recognizes the fact that the vast majority of rural residential streets are not posted, but also are not designed for the maximum unposted statutory speed limit of 55 mph. The ability to alter speed limits in these cases, without the need for a formal engineering and traffic investigation, will save substantial time and resources and is a reasonable application of engineering judgment allowed within the guidelines of the Manual on Uniform Traffic Control Devices.


The following additional policies shall be observed in establishing and posting speed zones:

A. Normally, isolated curves and turns, areas of restricted sight distance, nopassing zones, etc., will not be considered as areas for speed zones. Advisory speed signs may be used in connection with the appropriate warning signs at such locations, and such condition shall not be considered as influencing a lower limit for the entire zone.

B. Except on divided highways, different speeds for traffic in opposite directions shall not be posted.

C. A speed zone shall begin at the point or as near in advance of the point as possible where such speed zone is warranted and shall end at the point or as near to the point as possible where the zone is not warranted.

D. Minimum speed limits, when used, shall provide for an allowable speed range of not less than 10 miles per hour nor more than 15 miles per hour. No minimum speed of less than 30 miles per hour shall be used.

E. No sign in the speed limit series (R2) shall be mounted on the same post as another sign, except that the basic speed limit sign (R2-1) may be mounted below the town name sign (I1-I100 or I1-I101) or County Route Marker sign (M1-6).


A. The REDUCED SPEED LIMIT AHEAD sign (W3-5, W3-5A), shall be erected in advance of any speed zone that is 10 miles per hour or more under the limit in a preceding statutory limit.

B. The REDUCED SPEED LIMIT AHEAD sign shall be placed in advance of all altered speed zones in accordance with Table 2C-4, “Guidelines for Advance Placement of Warning Signs”, of the MUTCD and shall always be followed by a basic speed limit sign (R2-1) erected at the beginning of the zone where the altered speed limit applies. On divided and one way facilities having two or more lanes in one direction, signs shall be posted on both sides of the highway for all signs described above. Additional speed limit signs, as described in “D” below, in the same zone need not be posted on both sides of the road.

C. The difference in the speed limit from one altered speed zone to an adjacent altered speed zone shall not be more than 10 miles per hour.

D. Speed limit signs for altered zones shall be erected:

  1. At the beginning of each zone where the speed limit is altered.
  2. At the point of change from one zone to another.
  3. At additional locations within the zone where it is necessary to remind motorists of the limit that is applicable.

E. Signs shall not be erected until the speed limits are approved and are officially authorized.

F. The distance between basic speed limit sign installations in altered speed zones should be as follows:

  • Speed Minimum Maximum
  • Limit Distance Distance
  • MPH
  • 30 or less 660 feet or 2 blocks 1,320 feet or 4 blocks
  • 35 or 40 990 feet or 3 blocks 1,980 feet or 6 blocks
  • 45 or 50 1,320 feet or 4 blocks 2,640 feet or 8 blocks

G. The recommended distances between speed signs do not apply to the posting of statutory limits.

H. The size of speed limit signs shall conform with the National MUTCD. The size of the REDUCED SPEED LIMIT AHEAD and the first SPEED LIMIT sign should be based on the speed limit preceding the zone. The size of the remaining signs is based on the speed in the zone.

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