LADOT

TIPS & FAQ's

Traffic Management

Traffic Signal Timing and Coordination

Los Angeles Department of Transportation traffic signal timing engineers deploy the latest in computer technology to implement the most efficient timing that would reduce overall delay at a signalized intersection while maintaining "signal progression" to allow drivers to travel along major streets with least number of stops.

On a one-way street, this operation is relatively easy for Los Angeles Department of Transportation engineers. If it takes 10 seconds to travel from the first signal to the next, the second signal changes to green 10 seconds after the first one.

But when they re-time signals, LADOT engineers must coordinate traffic on the main street and side streets. They examine green light intervals from all directions at an intersection.

A traffic signal at a simple intersection has two phases—green light for the main street, followed by a green light for the side street. The main street’s green phase is generally much longer than the lower volume side street.

Signal timing adjustments must also accommodate devices at multi-phase intersections. Special devices such as left-turn arrows cause shorter green intervals for all other directions at the intersection, thus resulting in longer red intervals.


		Green times must be shared in a manner that balances the need for:
		Longer green phases for through traffic Safer, one-way traffic moves Avoiding delay for other motorists at the intersection

Signal progression on two-way streets is much harder to achieve. In order to keep traffic flowing without stops in one direction on a two-way, main street that has signals close together, motorists traveling in the other direction would have many stops.

Areas with many traffic signals are also difficult to control with signal progression. Here, two-way progression can only be accomplished for a few blocks at a time.


		In places where cross-streets must be synchronized with the main street, perfect signal timing is even more challenging. LADOT must consider:
		Main street progression Cross-street progression Pedestrian crossing time Traffic signal cycle lengths Travel speeds

Because signal progression is so complex, LADOT uses computer traffic models to minimize delay throughout the signal network. The models usually produce scenarios that result in the least total delay for all drivers, but may give less than perfect traffic progression. Typically, after LADOT engineers re-time traffic signals, the system-wide delay is reduced by 12 percent.

The Los Angeles Department of Transportation reviews traffic patterns periodically to adjust timing. LADOT also partners with neighboring cities to coordinate signals.

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