Traffic Signals

Complications that Impact Signal Coordination Plans

Traffic signal coordination plans are strongly influenced by dynamic conditions such as corridor speeds, traffic signal spacing, congestion, traffic volumes on major streets, pedestrian volumes, traffic signal cycle lengths, additional phasing, and safety considerations.

Each factor can significantly complicate good coordination schemes. Below are descriptions of these influencing factors, and the resulting conditions that may be undesirable for our driving public.

CORRIDOR SPEEDS
Signal coordination plans are established by using prevailing travel speeds. Motorists traveling at these speeds will achieve optimal travel times; however, those traveling above or below the prevailing speed may have significantly greater stops and delays as they are traveling outside the progression band.

TRAFFIC SIGNAL SPACING
Well-coordinated timings are established when signals are uniformly spaced along busy streets. For most busy corridors, spacing would be approximately ½ mile.

However, while newly developed arterial corridors provide signal spacing in accordance with access management policies, the older developed corridors do not have proper signal spacing which can result in more stops and delays .Traffic signal coordination plans are limited when it comes to signal spacing.

Signals are typically spaced no more than ¾ miles apart, as distance can cause the break up of platoons due to access movements, lane changes, truck traffic, varying travel speeds, geometric conditions and other elements. Without regulation, motorists may have more stops and delays than expected.

CONGESTION
Our plans are detrimentally impacted when capacities at our busiest intersections are exceeded. Under such conditions, traffic signal operations can not fully serve the demand, resulting in limited progression. In such cases, strategies may include serving only the heaviest directional flows.

TRAFFIC FLOW CHARACTERISTICS
Our signal coordination plans are strongly influenced by the volume of total traffic, the directionality of the traffic, and the amount of traffic entering, exiting or crossing from a side street. In most cases, our traffic signal coordination is designed to favor the heavier traffic flow. This may cause frustrations for motorists driving in less traveled directions as they may experience more stops and delays than desired.

PEDESTRIAN VOLUMES
We are very sensitive to the needs of pedestrians and bicyclists. To serve them safely, we have pedestrian signal phases at nearly all crossing locations. Though good for pedestrians, these phases reduce our proportional green time for thru-traffic on major streets. Reducing green “thru” bands affect coordination since it narrows the window when motorists can travel through the intersection without stopping.

TRAFFIC SIGNAL CYCLE LENGTHS
Traffic signals must operate under the same cycle length along a coordinated network to produce consistent results. These cycle lengths are typically set to serve the needs of the busiest intersection as well as provide the optimal coordination along the corridor.

As volumes grow on our major streets, cycle lengths increase. This is due primarily to the extended green phase times needed to serve the approach traffic demands. This may cause some delay at minor signalized approaches. In some situations, motorists traveling on side streets may experience longer delays than expected.

ADDITIONAL LEFT TURN SIGNAL PHASES
We are careful when adding left-turn phases along our busiest corridors due to their effect on green phase bands. Because our cycle lengths are fixed, each additional left-turn phase can reduce “thru” green times by as much as 25% to 40%. As a result, the reduced green “thru” bands can narrow the window allowing motorists to travel through the intersection without stopping.

SAFETY CONSIDERATIONS
We must address safety first when developing traffic signal timing plans. Each switch of the lights from green to red includes up to a five (5) second yellow phase and a two (2) second “all-red” phase. The all-red interval allows drivers to pass through the intersection when they are beyond the stop bar. The yellow interval provides additional time for motorists who are in the “dilemma zone” where decisions between stopping and proceeding through the signal are difficult to make. However, this phasing can adversely impact the green phase times on major and minor streets, widening progression bands.