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Old 05-09-2014, 03:30 PM
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Location: Long Island / NYC
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Originally Posted by impala096 View Post

To achieve good 2-way progression along a high speed arterial (45 mph or higher), signals really need to be spaced farther apart. Signals spaced 3/4 to 1 mile apart are much easier to provide good 2-way progression. Unfortunately, signals spaced 1/4 to a 1/2 mile apart are much more common along a suburban arterial as every school, residential drive, and shopping center request a traffic signal to be installed. You end up with a minefield of traffic signals, where drivers are inevitably going to get stopped by multiple red lights.
That would result in some rather long walks or detours for pedestrians, though.
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Old 05-09-2014, 03:56 PM
 
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There is a major arterial near me that has 20 traffic signals along a 2 mile stretch. Progression along this stretch of road is beautiful and you rarely get stopped at a red light. This is because of the 20 traffic signals, only 3 signals stop both directions of travel. When i say signals need to be spaced 3/4 to 1 mile apart to achieve 2-way progression, I'm really referring to signals that stop BOTH directions of travel (which admittedly i wasn't clear about).
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Old 05-10-2014, 08:59 AM
 
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An efficient high speed arterial where drivers rarely have to stop at a red light and plenty of access points for pedestrians to cross sounds like the best of both worlds.
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Old 05-10-2014, 01:14 PM
 
Location: Oakland, CA
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Originally Posted by impala096 View Post
An efficient high speed arterial where drivers rarely have to stop at a red light and plenty of access points for pedestrians to cross sounds like the best of both worlds.
That is terrible. It doesn't work at all. The pedestrians have to wait several minutes in an arterial since it is optimized for cars. They are at more risk when motorists are turning and traveling at high speeds.

Businesses lose because no one in a car can see them or make a last minute decision to stop since they are traveling at high speed.

That's a freeway with crosswalks.
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Old 05-10-2014, 03:00 PM
 
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^If you are saying that high speed arterials in general are terrible, I would disagree. They are a necessity for people living in car-centric suburbs that rely heavily on them to get around.

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The pedestrians have to wait several minutes in an arterial since it is optimized for cars.
Pedestrians have to wait to cross the street. What's your point though? Drivers have to wait to cross the street too.
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Old 05-10-2014, 09:46 PM
 
Location: Oakland, CA
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Originally Posted by impala096 View Post
^If you are saying that high speed arterials in general are terrible, I would disagree. They are a necessity for people living in car-centric suburbs that rely heavily on them to get around.



Pedestrians have to wait to cross the street. What's your point though? Drivers have to wait to cross the street too.
I work on a street like this. The lights are poorly timed. I can barely make it across the street before the light changes, and it is 3 minutes for the full cycle. I have been on worse streets where the full pedestrian cycle takes five minutes. Go ahead and set a timer for 5 minutes, and imagine waiting that long to cross the street.

This design is completely unsafe for pedestrians. More roads with slower speeds are way more efficient, you can avoid the congestion on the next block. And slower speeds make collisions safer for all people, in cars or not.

If there are no destinations on the road, or they are widely dispersed, high speeds make sense. It is useless when there are lots of driveways, shops etc. All of that slows traffic and makes your speed more variable which is slower.
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Old 05-10-2014, 11:05 PM
 
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Originally Posted by jade408 View Post
I work on a street like this. The lights are poorly timed. I can barely make it across the street before the light changes, and it is 3 minutes for the full cycle. I have been on worse streets where the full pedestrian cycle takes five minutes. Go ahead and set a timer for 5 minutes, and imagine waiting that long to cross the street.
That does seem ridiculously long. A signal running a 300 second cycle length is unheard of around here. Is there a legitimate reason why the signals run such long cycle lengths around you? This is a snippet taken from the FHWA Traffic Signal Timing Manual:

Quote:
In general, it is preferred that the cycle lengths for conventional, four-legged intersections not exceed 120 seconds, although larger intersections may require longer cycle lengths(9). Theoretically, intersection capacity increases as the cycle length becomes longer because a smaller portion of the time is associated with lost time. However, it is important to recognize that the improvements are modest and this assumes that all lanes are operating with saturated flows. This requires that turn bays are long enough to provide sufficient demand to a particular movement; auxiliary lanes are more likely to be blocked with longer cycle lengths. As shown in the figure, the change of cycle length from two minutes (120 sec.) to three minutes (180 sec.) results in a modest 2 percent increase in capacity. This calculation was made by estimating the start-up delay resulting from the signal changes, as it relates to the number of times that the signal intervals change during the course of an hour. The message conveyed by the information presented in Figure 6-16 is that one should avoid placing too much emphasis on longer cycle lengths as a panacea for congested conditions.
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Old 05-11-2014, 02:13 AM
 
Location: Oakland, CA
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Originally Posted by impala096 View Post
That does seem ridiculously long. A signal running a 300 second cycle length is unheard of around here. Is there a legitimate reason why the signals run such long cycle lengths around you? This is a snippet taken from the FHWA Traffic Signal Timing Manual:
I don't know but this street is a state highway, so the impacted municipalities have limited flexibility to make adjustments. The only priority is car speed and nothing else. It is a 6 lane road that runs roughly 40 miles through several cities. And for most it borders the downtown areas as well. It is used as an alternative to the north south freeway and runs parallel. The pedestrian experience is terrible on the full corridor.
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Old 05-11-2014, 05:14 AM
 
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Quote:
Originally Posted by impala096 View Post
^The above time-distance shows coordination for signals spaced 1/2 mile apart. There are two main reasons why the above time-distance isn't practical in the real world at this spacing.

#1. There is an associated "lost time" every time a traffic signal changes phases. Even for a simple 2-phase signal, an 80 second cycle would result in roughly 15% lost time each cycle. As the number of phases increase, the lost time per cycle increases even more. Essentially, an 80 second cycle length is too short to maximize the throughput of an intersection, even at a simple 2-phase signal.
The speed limit on Broad St. in South Philly is 25mph, the lights are ~140m apart and they're synced up with the one-way pairs that cross them so, if you're heading north for instance, you get two lights in front of you that turn green at the same time.

If there's no traffic (late at night) and I go 22mph I can get all greens for a distance of about 1.7 miles. Where I always seem to catch a red is where it crosses a second busy arterial that commands a lot more green time.
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Old 05-11-2014, 10:45 AM
 
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^The lights in downtown Portland are timed at 12.5 mph which provides great progression for drivers. Portlandís grid spacing is much smaller than South Philly (~80m vs. ~140m) so the lights have to be timed for a slower speed to maintain good grid-progression.

Portland's current grid (timed for 12.5 MPH) - YouTube

Any street with consistently spaced traffic signals can theoretically achieve perfect progression (by adjusting the speed limit and cycle lengths of the corridor). Of course my comments in this thread have focused on two-way high speed arterials which is a completely different animal than a downtown grid.
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