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Discussion Starter · #1 · (Edited)
Traffic signal progression is the concept of linking traffic signals together along a street so that "platoons" of vehicles can pass through signalized intersections without getting stopped at a red light. Some factors that affect traffic signal progression include:
-Signal spacing
-Speed of traffic
-Cycle length
-Roadway congestion

While good signal progression along a one-way street is easy to achieve it can be next to impossible along a two-way street where you often encounter inconsistently spaced traffic signals. Below are a few examples of streets with good 2-way progression:

DELAY = 0%

DELAY = 0%
 

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Cotton
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One of the key factors is how well-programmed the traffic signals are.

I saw probably the worst-programmed traffic signals in Rome: bus on my route stopped due to the red signal on every interchange, I caught red even on the interchange with minor road.
 

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Discussion Starter · #5 · (Edited)
Signal spacing and the design speed along an arterial are constants when a traffic engineer is trying to provide good progression along an arterial. The major tool in the toolbox is changing cycle lengths of the signals to provide good two-way progression. Here are the time-distance diagrams for a 70 second cycle length vs. a 140 second cycle length for two signals spaced 1/2 mile apart with a speed limit of 50 mph:


As you can see a 70 second cycle length provides near perfect dual progression while a 140 second cycle length provides the worst two-way progression possible. The problem is a 70 second cycle isn't a practical cycle length to run for a number of reasons.

1. During the heavy rush, even a simple 2-phased signal requires a longer cycle length to reduce delays. The shorter the cycle length, the greater percentage of time is dedicated to running the yellow and all red to satisfy the safety minimums each cycle.

2. Signals with long pedestrian crossings force the signal to run longer cycle lengths to satisfy MUTCD standards. It's not uncommon to have 130 feet long pedestrian crossings at major 6-lane boulevards with dual left turn lanes. With this length of crossing it requires 45 seconds of pedestrian times for each thru phase. Add to that the left turn phases and you're up to a 120 second cycle to satisfy the ped times.

A good case study to look at would be Beach Blvd in Huntington Beach, California. It's a major 6-lane boulevard with 50 mph speed limits that have signals stopping both directions of travel every 1/2 mile. Also, many of the pedestrian crossings are over 130 feet long (some over 150 feet) that force the signals to run high cycle lengths. Beach Blvd from the Pacific Ocean to the 405 is a great example of how not to design an arterial to achieve good two-way progression.
 

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Discussion Starter · #6 ·
Thanks tradephoric, I love these types of systems. When done correctly they are brilliant. When done not so correctly they're not so great
Here’s an example of a not so great system. This 6.4 mile stretch of Jefferson Ave. runs from downtown Detroit to Grosse Pointe Park and is dotted with traffic signals that increases travel times. This road is a main route for anyone living in Grosse Pointe who works in downtown Detroit:

Road: EB Jefferson Avenue (Griswold St. to Alter St.)
Date: Thursday, July 14, 2011 (4:20-4:35 PM)
Distance: 6.43 miles
Speed Limit: 35 mph
Freeflow travel time: 11.0 minutes
Actual travel time: 15.4 minutes
Delay = 40%


Road: WB Jefferson Avenue (Alter St. to Griswold St.)
Date: Thursday, July 14, 2011 (4:35-4:50 PM)
Distance: 6.43 miles
Speed Limit: 35 mph
Freeflow travel time: 11.0 minutes
Actual travel time: 16.1 minutes
Delay = 46%


I try to record drives on days with light traffic so that any delays are solely due to the traffic signal timings. Roadway congestion is listed in the first post as a factor affecting traffic signal progression. Basically if it's taking multiple cycles to pass through a traffic signal then progression has already been lost.

Above each video in bold is the amount of delay experienced due to the traffic signal timings. A 0% delay would indicate you were able to average the posted speed limit and make it through every green light. A 40% delay would indicate a drive with a free-flowing travel time of 10 minutes would take you 14 minutes to complete, with 4 minutes of delay as a result of the signal progression.
 

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Discussion Starter · #7 ·
An example of a road with good signal progression in only one direction of travel:

Road: NW Dixie Hwy (Telegraph to White Lake Rd.)
Time: Thursday @ 4:00 PM
Distance: 6.8 miles
Speed Limit: 45 mph
Freeflow travel time: 9.1 minutes
Actual travel time: 9.1 minutes
Delay = 0%


Road: SE Dixie Hwy (White Lake Rd. to Telegraph)
Time: Thursday @ 4:15 PM
Distance: 6.8 miles
Speed Limit: 45 mph
Freeflow travel time: 9.1 minutes
Actual travel time: 15.2 minutes
Delay = 67%
 

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Discussion Starter · #8 ·
Here’s a quote in a report produced by the FHWA entitled “Traffic Signal Timing Manual” June 2008.

“It became clear that for safety and liability reasons, agencies must en sure a basic level of operation of the traffic signal system so that signals continue to turn green, yellow and red. The signals may not function efficiently for traffic or pedestrians, but, technically, the signals are working and that is what people see. However, the uniformly low scores (on the National Report Card) indicate that, for the most part, people consistently experience poor traffic signal performance and, as a consequence, their expectations are low. The pattern, once again, is one where agencies are forced to use their resources to deal with critical maintenance issues when they arise rather than proactively. Signal systems are managed to simply ensure base levels of performance.”
In a nutshell the general public is use to inefficient and poorly maintained traffic signals. A one-way street should be the easiest type of road to provide good progression but as you can see in the below video this is not always the case:

Road: Cass Avenue (Johnson Ave. to Huron St.)
Date: Thursday @ 4:30 PM
Distance: 0.60 miles
Speed Limit: 35 mph
Freeflow travel time: 62 seconds
Actual travel time: 187 seconds
Delay = 202%


This one-way street might have great progression assuming the signals are running per the traffic signal timing plans. However, a poorly maintained system can quickly turn a road with good progression into one with the very bad progression. Here are some factors that can effect the progression along a corridor:

  • Signal out of step due to emergency preempt being activated.
  • Signal out of step due to pedestrian actuation.
  • Clock in controller is wrong (storm rolled through, bad power in area causing clocks to drift, GPS unit not acquiring data).
  • Signal out of step due to Dial change in controller.
  • Broken detection causing side-street to cycle and run long.
  • Signal timing entered into the controller incorrectly.
  • Traffic signals along a corridor have different controller types which can lead to clocks drifting at a different rate.
  • Signal heads turned during a wind storm and now the side-street heads are facing the main-street direction (rare, but quite unsafe when it occurs).
 
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