Most American cities use the recommendations of The Traffic Engineer’s Handbook, which presumes an average speed of 6 feet per second for Joe Pedestrian.
If you would like this expressed as a formula, take the total number of feet across the street and divide by 6. Subtract that figure from the total amount of time that the light will be green.
That’s how long your Walk signal should last. So if the street is 42 feet wide and your green light in that direction is 30 seconds long, the Don’t Walk signal should start flashing at about the 23-second mark.
That is the case in most intersections, but not all of them.
The Handbook recommends that in areas with a high number of senior citizens, traffic engineers should lower their pedestrian speed estimate to 4 feet per second.