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53 8 Additional Signal Features 8.1 Signal Cycle Length Cycle length must take into consideration the traffic signal phase mode and sequence. Shorter cycle lengths are generally considered more efficient, resulting in less delay and shorter queues; however, longer cycle lengths may be necessary to accommodate protected turning and/or pedestrian movements. FHWAâs Signalized Intersections: An Informational Guide states that âcycle lengths for conventional, four- legged intersections should not exceed 120 seconds, although larger intersections may require longer cycle lengths of 140 to 150 seconds.â Signalized Intersections: An Informational Guide provides additional information on the various aspects that determine a signal cycle length. 8.2 Actuation (FHWA 2005) 8.2.1 Pretimed Pretimed signalization is commonly found in grid systems with very closely spaced signals. A grid benefits the most from its intersections being pretimed. Pretimed signalization is also applicable on arterial streets within coordinated networks if the side street volumes are consistently high. Pretimed signalization is best suited for locations where the traffic follows a predictable pattern over a long period. 8.2.2 Fully Actuated Fully actuated signalization should be used at isolated signals or signals where there is no coordination within the network. Due to its high level of responsiveness, fully actuated signalization is usually the most efficient and should be considered first. For this mode of operation, sufficient well-maintained vehicle and pedestrian detection is needed. 8.2.3 Semi-Actuated Semi-actuated signalization is desirable where the main line volumes are much larger than the side street volumes, whether the signal is coordinated or not. Semi-actuation can also be used in grid scenarios to call for certain movements that are sometimes skipped, such as left turns. 8.2.4 Nth Car Actuation It is customary for a detector to place a call to the controller if a single vehicle is waiting to make an actuated movement. However, detectors for protected-permissive left turns can also be configured to serve a protected phase only when a certain number of vehicles are queued. As an example, many signals in the Phoenix metropolitan area operate with third-car actuation (or first- and third-car actuation), where a protected phase only appears if at least three vehicles are queued for the movement. If fewer than three vehicles are queued, they typically make a permissive turn. Nth car actuation reduces the delay to conflicting phases by omitting protected left-turn phases when volume is low enough to be handled on a permissive phase. While studies have shown this type of actuation may not be advantageous during peak hours when multiple left-turning vehicles are present during all cycles, this may offer an operational advantage during off-peak hours.
54 8.3 Coordination Coordination between signal timings at adjacent intersections along a corridor allows for smooth flow of traffic and minimizes the delay and queuing along the corridor. Additionally, coordination creates platooning of the mainline through traffic, creating additional gaps for mainline left/U-turning traffic to complete the left-turn and U-turn movements. Coordinated signals usually share a common cycle length. They are best used in networks of relatively closely spaced signals where the traffic characteristics (such as queuing) of one signal impact adjacent signals. The MUTCD recommends that signals should be coordinated when they are within ½ mile of each other (FHWA 2009b), but this language should not be read to imply that signals farther than ½ mile apart should not be coordinated. On the contrary, it is often beneficial to coordinate signals when they are a mile or more apart. While coordinated signals usually share a common cycle length, it is also possible to coordinate signals when one signalâs cycle length is an even multiple of another. For instance, a signal with low cross traffic may operate at exactly half the cycle length of a nearby high-volume intersection. This strategy sometimes helps balance delays and queues at intersections in a coordinated network with very different traffic volume patterns. 8.3.1 Uncoordinated Uncoordinated signals operate with independent cycle lengths. They are best used at fully actuated intersections far from other signals. Signals often are in coordination during parts of the day with relatively high traffic volumes and uncoordinated at lower-volume times, often overnight and/or on weekends.
