Signal Safety Interval Calculator
This module calculates the recommended Change (Yellow) and Clearance (All Red) intervals to be used at a signalized intersection.
- The Change Interval is used to alert motorists that their signal will soon become red and allows them adequate time to determine if they can traverse the intersection or if they must stop.
- The minimum Change Interval is 3 seconds.
- The maximum Change Interval is 6 seconds.
- The Clearance Interval is the time when all approaches have a red signal, allowing any vehicles remaining in the intersection from the previous Change Interval to exit.
Properly designed intervals allow for safer intersection operations.
This module does not calculate the recommended green time, which depends on actual traffic volumes.
- All green boxes must have data entered for the calculator to function.
- In the first box, enter the speed of the approaching vehicle. This is usually the measured 85th-percentile speed of traffic on that approach. If this information is not available, use the posted speed limit.
- In the second box, enter the width of the intersection. This is measured from the STOP line to the outside edge of the farthest crossing travel lane or if pedestrians are present to the far side of the farthest conflicting pedestrian crosswalk along the actual vehicle path. A wider intersection will result in longer clearance times.
- In the third box, enter the grade of the approach. Upgrades are positive, downgrades are negative. Downgrades will result in longer times.
- In the fourth box, select a design vehicle. By default, the calculator assumes a passenger car (20-ft length). Other choices are an intercity bus or WB-62 tractor trailer. An Other box with manual length entry is also available. Larger vehicles will result in longer clearance times.
- Press the "Calculate" button once all the fields have been completed.
- The Clearance Interval (All Red) will appear in the left box next to the Red signal.
- The Change Interval (Yellow) will appear in the right box next to the Yellow signal.
- To remove all entries and calculations, click the "Reset" button.