Instructions Text

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.

  1. All green boxes must have data entered for the calculator to function.
  2. 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.
  3. 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.
  4. In the third box, enter the grade of the approach. Upgrades are positive, downgrades are negative. Downgrades will result in longer times.
  5. 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.
  6. Press the "Calculate" button once all the fields have been completed.
  7. The Clearance Interval (All Red) will appear in the left box next to the Red signal.
  8. The Change Interval (Yellow) will appear in the right box next to the Yellow signal.
  9. To remove all entries and calculations, click the "Reset" button.

​Pedestrian Intervals

This module calculates the recommended Walk and Clearance (flashing DON'T WALK) intervals for pedestrian signals. These values can be used for separate pedestrian phases or to help in establishing a minimum green time when used in conjunction with vehicular movements.

  1. Select a walking speed from the drop-down menu.
    1. Normal (Default) = 3.5 feet-per-second
    2. Elderly/Children = 3.0 feet-per-second
  2. Enter the width of the roadway to be crossed, either curb to curb (if provided) or shoulder to shoulder
  3. Press the "Calculate" button
  4. The internal times (in seconds) will appear in the white boxes on the right
    1. The top box shows the interval length for the WALK symbol
    2. The middle box shows the interval length for the DON'T WALK (flashing) symbol
    3. The bottom box shows the total pedestrian phase interval (WALK + DON'T WALK + 3-seconds for buffer interval)
  5. To remove all entries and calculations, click the "Reset" button.

The minimum pedestrian walk interval is 7 seconds.

The clearance interval is the time required for a pedestrian to cross from one side of the road to the other if they leave the sidewalk at the beginning of the clearance (flashing DON'T WALK) interval.

The Buffer interval can be concurrent with the adjacent vehicular yellow interval.