Untangling gridlock: four examples of new technologies at use on L.A. city streets. (Up Front).

Left Turn Arrow

Concept: There are two kinds: the "protected" green turn arrow, where all other signals are red, and the "protected permissive" green arrow. Protected greens give priority to turning and straight-ahead traffic moving in the same direction. They are placed most often in intersections that have a history of accidents caused by left turns or two left-turn lanes. Left turn green arrow is followed by red arrows, halting turns.

At locations with no history of accidents but heavy demand for left turns, the protected permissive arrow appears when three or more cars are waiting. After the arrow goes off, left turns are allowed on the dominant green light. How it works: For protected green arrows, street sensors detect a single car waiting and transmit a message over fiber optic lines to the signal control box on the sidewalk. At protected permissive green arrow lights, sensors placed 60 feet or further behind the crosswalk activate a green arrow if three or more cars are waiting to turn. All control boxes are connected to a regional communication hub at a city facility by fiber optic lines. The hubs transmit the data to the central mainframe computer, which displays the data on PCs. Engineers can check live traffic conditions, view changes, and determine if a manual override is needed.

History: Street sensors tracking congestion were first installed near Los Angeles Memorial Coliseum for the 1984 Olympics. There are now more than 12,000 sensors. Automatic left turn function began in early 1970s. Where: Most common along Wilshire Boulevard with double left turn lanes and busy intersections with good visibility such as 3rd Street and Vermont Avenue.

Effectiveness: The city says the installation of left turn arrows has reduced delays at intersections by 32 percent.

Bus Priority

Concept: Keeping Metro Rapid buses on schedule.

How it works: Each bus has a transmitter containing schedule information that notifies street sensors through a radio signal as it passes over them. Sensors communicate data to the mainframe computer. If bus is behind schedule, upcoming traffic lights automatically stay green up to 10 seconds longer or are changed to green.

History: Began in June 2001.

Where: Wilshire Boulevard and Ventura Boulevard routes. Now includes Van Nuys Boulevard, Florence Avenue and San Fernando Valley East-West Bus Rapid Transit.

Effectiveness: Ventura Boulevard routes are running 25 percent faster.

Automatic Signal Synchronization

Concept: To ease flow by limiting stop-and-go traffic between intersections on busy streets.

How it works: About 2,000 of L.A.'s 4,200 traffic signals rely on preprogrammed synchronization. At the intersections of busy thoroughfares and secondary streets, sensors on side streets detect a car approaching and transmit a signal change command to the mainframe, which notifies the control box at the intersection. In these cases, signal changes synchronize with signals on the busier street.

History: Side street sensors were first installed in the 1960s. Prior to 1996, all traffic signal control boxes had pre-timed signal plans that did not adapt to live conditions. Since 1996, new software measures live traffic and makes signal changes automatically. About 1,000 signals are on this new system. The mainframe was originally installed in 1984 with software made by the federal government.

Where: Sections of city with many adjacent, high-congestion intersections like Wilshire Boulevard and downtown. Signals around Staples Center, Hollywood & Highland, Victory and Ventura boulevard corridors in the San Fernando Valley.

Effectiveness: The frequency of stops at red lights has been reduced by 30 percent since the 1984. But traffic light timing can be thrown off by signals not yet connected to the mainframe computer. Synchronization tends to slip over time.

Manual Control

Concept: Adjusting traffic controls to account for special events or abnormal traffic conditions.

How it works: Street sensors instantly transmit traffic data to signal control box, which notifies the central mainframe computer and displays live results on PC. Commands are typed into PC and routed back to signal control box. History: Mainframe computer has been able to make changes to L.A. City traffic signals since 1984. Software was upgraded in 1997 to allow control from a PC.

Where: Around public venues like Staples Center. Areas with frequent traffic jams.

Effectiveness: Automatic, signal timing plan using PC system saves engineers time to deal with special events or unusual traffic situations.