There are many different types of traffic signals and uses for them. Traffic Engineering follows the Manual on Uniform Traffic Control Devices (MUTCD) and industry guidelines for placement and other characteristics of traffic signals. The progression of vehicles along a roadway is a guiding factor in defining how the signal will operate. In general, traffic signals are timed to reduce the delay for the most vehicles and help traffic flow more smoothly.

A standard for most signalized intersections is that:

• collector streets, which enter onto or off of major arterial streets, have shorter green light cycles because of lighter traffic volume, while;

• major arterial streets have longer green light cycles to move a greater volume of traffic efficiently.

This signal timing minimizes the delay for the traffic on the main arterial but sometimes causes larger delays for the traffic on the minor streets. If signals are timed correctly and the appropriate type of signal controller is used, minimum total delay (for major and minor street traffic) should be achieved.

Signal timing and trigger mechanisms used to move traffic through signalized intersections are as follows:

Pre-timed signals are those signals that have a preset cycle length for specified times of the day or for the entire day. Unlike actuated signals, a pre-timed signal cannot adjust to traffic flow. Therefore, the optimum cycle lengths for these intersections must be determined. Factors considered in designing a cycle length include the number of phases, the largest number of vehicles that can use a green light, and the number of lanes having right-of-way, just to name a few. Pedestrians are also a major consideration and their presence could increase the side-street green times and consequent cycle length at an intersection.  All of the downtown signals in Davenport and a few outside of downtown are pre-timed signals.

Some pre-timed traffic signals are timed as a system.  This synchronization of signals is often implemented on major thoroughfares where space between signals is close. Synchronization on major thoroughfares moves traffic more efficiently with generally fewer stops.   When a system is synchronized if you hit a green light at the first signal in the system and you are traveling the speed limit you will hit another green light at the next intersection, and so on.  

Actuated signals are those signals that can vary their cycle length (i.e., they vary the length of the green lights given) to the traffic flow that uses the intersection. Detectors register vehicles that approach these intersections and send the information to a signal controller. The controller adjusts the length of green light for the current traffic conditions. There are two types of actuated signals: fully actuated and semi-actuated.

Fully actuated signals are found at intersections that exhibit large fluctuations of traffic volumes from all approaches during the day. There is a set minimum and maximum green time for these signals. If there are no opposing vehicles stopped at the intersection, the moving traffic will receive additional green time. The minimum green time is often set equal to the time required for a pedestrian to safely cross intersection.

Semi-actuated signals have detectors only on the minor street approaches to an intersection. They are often found at the intersections of main arterials and minor low-volume roadways. The main arterial has a green light until a vehicle is detected on the minor street. When the traffic volume is high on the side streets (during peak travel times) the semi-actuated signal acts as a pre-timed signal.

Citizens often ask how placing a traffic signal is determined. Traffic engineers cannot simply install traffic signals where they are not warranted. Traffic engineering studies, using MUTCD* criteria, must be performed prior to installation of traffic control devices at new location(s) and/or prior to modification of traffic control device(s) at existing locations.

When considering new or changing a traffic control device, traffic engineers evaluate the objectives of the proposed installation or change and apply engineering judgement based on MUTCD standards and goals. According to the MUTCD, traffic engineers must assess these four goals when allocating the right-of-way to traffic:

1. Moving traffic in an orderly fashion,
2. Minimizing delay to vehicles and pedestrians,
3. Reducing crash-producing conflicts, and
4. Maximizing capacity for each intersection approach.

*Manual on Uniform Traffic Control Devices

Crossing a signalized intersection can be dangerous for a pedestrian. It is important for an intersection with regular pedestrian traffic to have clearly visible signals that tell the pedestrian when they should not cross the intersection.

This is especially necessary when the pedestrian cannot see the green signal for the vehicles travelling in the same direction. The presence of pedestrians can greatly affect traffic signal timing. Understanding how signals work is very important to pedestrian safety.

What do the phases of walk/don't walk signs mean?

• When the WALK indication is illuminated, the pedestrian may start walking across the intersection.  The pedestrian should still check for potential conflicts with turning vehicles.
• When the DON’T WALK indication is flashing, it is not safe for a pedestrian to start crossing. A pedestrian who is already crossing when the indication starts to flash should have sufficient time to safely finish.
• When the DON’T WALK sign is constantly illuminated, it is not safe for a pedestrian to be in the crosswalk.

Why does the walk signal stay on for only a few seconds?
A major misconception by pedestrians is that they should not be in the crosswalk when the WALK sign changes to a flashing DON’T WALK sign. Some pedestrians actually turn around and go back to where they started! Pedestrians should realize that the only time that they should not be in the crosswalk is the period when the DON’T WALK signal is illuminated constantly. They should not start crossing if DON'T WALK is flashing. Most WALK phases last only about four to seven seconds and are normally referred to as the minimum start‐up time for a pedestrian. The flashing DON’T WALK phase should give a pedestrian enough time to safely cross the intersection.

How is the pedestrian crossing time related to signal timing? 
There is a simple equation that transportation professionals use to determine how much time is needed for a pedestrian to safely cross an intersection. The equation is GP = (4 to 7) + (W/3.5), where GP is the minimum pedestrian crossing time needed in seconds. In general, the WALK signal is illuminated for four to seven seconds (depending on the level of pedestrian flow). The flashing DON’T WALK sign is illuminated for (W/3.5) seconds, where W is the distance in feet from the curb to the center of the farthest lane on a roadway and the 3.5 (in feet per second) is the walking speed typically assumed for pedestrians.

There are also other methods to signal timing for pedestrians. A common alternative to the previous approach is the installation of a pedestrian push‐button detector. When the button is pushed, the traffic signal controller provides a minimum crossing time for the pedestrian during the next available concurrent vehicle green phase. Because of the existence of a pedestrian, the green time might be increased past what is needed for the vehicles, and the percentage of the cycle allocated to the other phases decreased proportionally. Sometimes it is assumed that a pedestrian crosses an intersection in two trips.  In this case, pedestrian timing is based on the use of a median island.

What determines the placement of a pedestrian signal? 
The safety of pedestrians crossing intersections is very important. The Manual of Uniform Traffic Control Devices(MUTCD) outlines several guidelines that address pedestrians at signalized intersections. The following are some of the basic conditions that stipulate the placement of a pedestrian signal:

• When pedestrian volume or school crossing warrants require the installation of a traffic signal;
• When there is an exclusive interval set for pedestrian crossing;
• When traffic pattern indicators are not visible to pedestrians (e.g., one‐way streets);
• At established school crossings where the intersection may be signalized under any warrant;
• When the pedestrian volume is large enough to cause vehicle-pedestrian conflicts and there needs to be a longer pedestrian clearance interval;
• When multi-phase indications confuse pedestrians; and
• When pedestrians cross only part of a street (to or from an island).