- Actuated signals respond to the traffic present at the intersection, so that the pattern of the signal (the length and order of each phase) depends on the traffic and can be different at every cycle.
- Sensors are installed in actuated lanes which report to the signal computer; green is provided for those lanes only when traffic is present and only until the traffic has vacated those lanes or the maximum time set for that phase has been reached.
- Therefore, signal timing is not given to vehicles or pedestrians in actuated lanes / crosswalks unless the signal computer detects their presence. Detection can be accomplished as follows:
- Drivers who want to cross are detected when their vehicle triggers sensors, which may be in the street or mounted overhead.
- Pedestrians who want to cross are usually detected when they push a button (but many actuated crossings provide no button, so there is no way to indicate a desire to cross and be provided with enough time!). At some intersections, the presence of pedestrians is detected with passive detectors such as cameras.
- Once a vehicle is detected on the minor street trying to cross the major street:
- as soon as the major street has its minimum time for green light, the major street gets a yellow / red signal (if the major street already had its minimum time before the vehicle was detected on the minor street, the signal for the major street immediately turns to yellow / red);
- the signal for the minor street then changes to green for just enough time to allow vehicles from the minor street to cross the intersection (usually 7 seconds allowed for the first car, then 3 additional seconds for each remaining car detected).
Note that this is rarely enough time for pedestrians to cross the major street before the onset of perpendicular traffic.
- Most actuated signals are semi-actuated, so that the major street has the green light endlessly unless the signal computer detects a vehicle or pedestrian wanting to cross the major street.
While the major street has the green light for vehicles to cross the minor street, there may also be a WALK sign for pedestrians wanting to cross the minor street but, if not, the WALK signal for crossing the minor street can be actuated by pushing a button.
- Click here for signs of actuation.
The following videos were taken at an actuated signal for crossing Piney Orchard at Waugh Chapel. The crosswalk for Piney Orchard is the width of 6 lanes, which would take the average pedestrian about 20 seconds to cross.
STRATEGIES TO DEAL WITH / ADAPT FOR ACTUATION:
The first video shows that when the pedestrian button is not pushed, the signal stays green only long enough to let the vehicles cross Piney Orchard, which is about 13 seconds because there were a half-dozen vehicles waiting to cross.
If we had started to cross when the parallel traffic surged forward and we walked the speed of the average pedestrian, we would have just passed the median and started the second half of the street when the signal would have changed to red.
The second video shows that when we push the pedestrian button, the WALK signal comes on and we have more than 30 seconds to cross.
Return to Teaching Blind Pedestrians to Cross at Complex Signalized Intersections
- Determine if crossing may be actuated (see signs of actuation). If so, find and push the pedestrian button, and cross when pedestrian signal indicates "WALK."
Nonvisual cues to the onset of the WALK signal:
- Usually, after the button has been pushed, the WALK signal starts the next time that traffic in the nearest parallel lanes goes straight-through (see exceptions).
"Traffic in the nearest parallel lanes" means (see illustration at Complex Traffic Patterns):
- the southbound traffic when crossing to/from SW and NW corners;
- the north-bound traffic when crossing to/from NE and SE corners.
- DO NOT use timing to predict signal phases! click here for more information
- NOTE: If the crossing is actuated but there is no pedestrian button, there is risk that before you can finish crossing, the traffic on the street you’re crossing will have their green signal.
Return to home page