(A) Intent and purpose.
(1) The following section pertains to the clear line of sight at intersections of streets and major driveways. The criteria are based on American Association of State Highway and Transportation Officials (AASHTO) and the Illinois Department of Transportation (IDOT) design procedures. This section has been adopted to provide for safe turning movements by providing clear visibility zones at intersections clear of obstructions such as trees, walls, signs, buildings, etc. This policy is not intended for determining traffic controls at intersections. The need for traffic controls is to be assessed by the rules and regulations contained in the Manual on Uniform Traffic Control Devices and as recommended by the Village Engineer.
(2) The application of intersection sight distance and stopping sight distance shall extend to all proposed and existing public street intersections and all proposed and existing intersections of private streets and drives with public streets, including multi-family and commercial entrances. This also includes local to local intersections such as those within proposed subdivisions. The village requires that both intersection and stopping sight distances shall be shown with their geometries on all final construction drawings and be analyzed during the traffic impact study process.
(B) Visual obstructions. Visual obstructions shall be defined as any object within intersection sight triangles between the heights of no more than three feet measured relative to the elevation of the nearest pavement crown. This is based on a driver eye height of three feet and an object height of three feet (AASHTO and IDOT). Overhanging branches or other elevated obstructions may not be any lower than ten feet measured relative to the elevation of the nearest pavement crown.
(C) Stop sign controlled intersections. In determining the area of the sight triangle for intersections with stop sign control, the following criteria shall be used:
(1) One-way stop control: The sight triangles for a stop controlled minor road at a T-intersection with left turns allowed shall be the same as for the two way stop control.
(2) Two-way stop control: The sight triangles for a stop controlled minor road with left turns allowed shall be based on Figures 1 & 2 and Tables 1 & 2. The left turning movement requires a longer intersection sight distance than the right turning movement; therefore, the criteria for the left turn conditions apply.
(a) Figure 1. Left turners looking left.
(b) Figure 2. Left turners looking right.
(c) Figure definitions and calculations.
1. ISD = Intersection Sight Distance. See table 1 and table 2.
2. d = Distance from the edge of nearest through lane to the decision point; 18 ft. preferred, 14.5 ft. minimum (AASHTO 2004). Any distance other than 18 ft. must be approved by the Village Engineer.
(d) Table 1.
Table 1. Intersection Sight Distance for Passenger Cars at Stop Controlled Intersections - Left turners looking left | ||||||||
Design Speed of Major Road (mph) | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 55 |
ISD: Intersection Sight Distance (ft) Based on the equation below | 225 | 280 | 335 | 390 | 445 | 500 | 555 | 610 |
(e) Table 2.
Table 2. Intersection Sight Distance for Passenger Cars at Stop Controlled Intersections - Left turners looking right | ||||||||
Design Speed of Major Road (mph) | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 55 |
ISD: Intersection Sight Distance (ft) Based on the equation below | 225 | 280 | 335 | 390 | 445 | 500 | 555 | 610 |
(f) Table definitions and calculations. Based on AASHTO - Geometric Design of Highways and Streets (2004) and IDOT BLR Sections 28 & 36.
1. Intersection Sight Distance (ISD) = Design speed (mph) x 1.47 x time gap (sec)
2. Intersection Sight Distance (ISD) - length of the leg of the sight triangle along the major roadway (ft)
3. Design speed of the major roadway (typically 5 mph over the posted speed limit)
4. 1.47 - conversion from mph to fps
5. Time gap is (7.5 sec - passenger car; 9.5 sec - single-unit truck; 11.5 sec - tractor/semitrailer) for a turning vehicle to enter a 2-lane major road without a median from a minor road with 3% or less grade. If the major road is multi-lane or has a median, add 0.5 sec per additional 12 feet crossed to turn left. If the minor road grade is >3%, add 0.2 sec per percent grade above.
(3) Right out only, stop control: The sight triangle for a stop controlled intersection with only right turns allowed from the minor road shall be based on Figure 3 and Table 3.
(a) Figure 3. Right turners looking left.
(b) Figure definitions and calculations.
1. d = Distance from the edge of nearest through lane to the decision point; 18 ft. preferred, 14.5 ft. minimum (AASHTO 2004). Use 15 ft. unless otherwise approved by the Village Engineer.
(c) Table 3.
Table 3. Intersection Sight Distance for Passenger Cars at Right Out Only - Right turners looking left | ||||||||
Design Speed of Major Road (mph) | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 55 |
ISD: Intersection Sight Distance (ft) Based on the equation below | 195 | 240 | 290 | 335 | 385 | 430 | 480 | 530 |
(d) Table definitions and calculations. Based on AASHTO - Geometric Design of Highways and Streets (2004) and IDOT BLR Sections 28 & 36.
1. Intersection Sight Distance (ISD) = Design speed (mph) x 1.47 x time gap (sec)
2. Intersection Sight Distance (ISD) - length of the leg of the sight triangle along the major roadway (ft)
3. Design speed of the major roadway (typically 5 mph over the posted speed limit)
4. 1.47 - conversion from mph to fps
5. Time gap is (6.5 sec - passenger car; 8.5 sec - single-unit truck; 10.5 sec - tractor/semitrailer) for a turning vehicle to turn right from a minor road with 3% or less grade. If the minor road grade is >3%, add 0.2 sec per percent grade above 3.
(4) All-way stop control: The first stopped vehicle on one approach should be visible to the drivers of the first stopped vehicles on each of the other approaches.
(D) Traffic signal controlled intersections. The sight triangles for a traffic signal controlled intersection shall be described by the following conditions at each approach;
(1) Signals with off peak or night time flash operations shall follow the criteria for two way stop sign control on the red flashing approaches (Figures 1 & 2 and Tables 1 & 2).
(2) Signal approaches with right turns on red allowed that are not defined by IIA, shall follow the criteria for right out only, stop control (Figure 3 and Table 3.)
(3) Signal approaches not described by the conditions in either IIA or IIB shall use an intersection sight distance based on all-way stop control (ID).
(E) Yield controlled intersections. The sight triangles for a yield controlled intersection shall be described by the following conditions at each approach. Please note this information is referenced from the Guide for the Development of Bicycle Facilities, AASHTO, 4th Edition, 2012.
(1) Intersections that are not controlled by stop signs or signals are to be considered as yield-controlled intersections.
(2) For yield-controlled intersections between multi-use paths and roadways use the following figures/tables to calculate the appropriate sight triangles. Approach sight triangles depend on the design speeds of both the path and the roadway. If yield control is to be used for either approach, it is desirable that available sight distance be adequate for a traveler on the yield-controlled approach to slow, stop, and to avoid a traveler on the other approach. The roadway leg of the sight triangle is based on bicyclists' ability to reach and cross the roadway if they do not see a potentially conflicting vehicle approaching the roadway, and have just passed the point where they can execute a stop without entering the intersection. Figure 5-15, Table 5-7 (For Roadway Leg), and Table 5-8 (For Path Leg) shall be used for this condition.
(3 ) For an uncontrolled intersection of a shared use path with a walkway (i.e, under yield control), a clear sight triangle extending at least 15 feet along the walkway should be provided (see Figure 5-16).
(4) If a shared use path intersects with another shared use path, sight triangles should be provided similar to a yield condition at a path-roadway intersection. However, both legs of the sight triangle should be based on the stopping sight distance of the paths. Use the equation in Table 5-7 for both legs of the sight triangle.
(Ord. 15-1229, passed 11-18-15)