What is "perfect" signal timing?

[tradephoric]

While short cycle lengths are desirable, ensure that cycle lengths are long enough for pedestrians to cross wide streets in a single leg without getting stuck in the median, unless the median is a destination in and of itself. 

The problem is pedestrians crossing Woodward get stuck in the median.  If you re-time the lights so they could make it all the way across, the cycle length would balloon to about 180 seconds.  But now pedestrians who don't want to wait long to get a Walk are waiting at a signal with a 180 second cycle length... Houston, we have a problem.

The solution to this is to make the median a destination. Widen out the median and construct some buildings in the middle.  It's called a Town Center Intersection (TCI) and unlike a Median U-turn you don't have to take a turnaround to complete your left turn.  It's just simple and effective.   Also, in many cases the pedestrian crossings are left to the outside of the box, so in theory pedestrians shouldn't conflict with left turning vehicles (which can be a dangerous vehicle/pedestrian conflicts at an intersection).

https://www.youtube.com/watch?v=rCQn89wItTY

[tradephoric]

Here are 3 SYCHRO models of a conventional intersection, Median U-turn, and Town Center Intersection.  Each has the same traffic volumes and upstream geometry (two intersecting 6-lane roads).  Backups abound at the conventional intersection with protected left turn phases (even with dual left turn lanes).  A 4-phase intersection will never have the same capacity as a simple 2-phase intersection.

https://www.youtube.com/watch?v=dZC0VeWo8IY

https://www.youtube.com/watch?v=6Z1NhXYLq9Y

https://www.youtube.com/watch?v=RWIHiNKJ26c

[mrsman]

The solution to this is to make the median a destination. Widen out the median and construct some buildings in the middle.  It's called a Town Center Intersection (TCI) and unlike a Median U-turn you don't have to take a turnaround to complete your left turn.  It's just simple and effective.   Also, in many cases the pedestrian crossings are left to the outside of the box, so in theory pedestrians shouldn't conflict with left turning vehicles (which can be a dangerous vehicle/pedestrian conflicts at an intersection).

https://www.youtube.com/watch?v=rCQn89wItTY

In a sense, these town center intersections are like the intersection of 2 one-way pairs.

Here is an example in Laurel, MD of an intersection of two relatively important highways, US 1 and MD-198.  Each road expands to one-way pairs for the intersection, as well as for several blocks thereafter.  This scenario is far more efficient than a standard intersection with left turn arrows and the like.  If you zoom in, you will see special channelization for some of the turns between the one-way segments.


https://www.google.com/maps/place/Laurel,+MD/@39.101069,-76.8508037,15z/data=!4m5!3m4!1s0x89b7dd10e849dcd1:0x98cd5e2efdcd92fc!8m2!3d39.0992752!4d-76.8483061

[mrsman]

Achieving good signal progression in both directions of travel along a 2-way road comes down to three main variables…. signal spacing, cycle length, and speed.  The posted speed of Woodward is 45 mph and the signals are spaced roughly 3,000 feet apart.  If the speed and signal spacing along a corridor stays consistent you can figure out the cycle length that will achieve good signal progression in both directions.  I put together a chart that figures out the resonant cycles for different speeds and signal spacing.  In the case of Woodward the signals need to run roughly 90 seconds to achieve good signal progression in both directions of travel (the corridor is actually timed for about 48 mph, but most people are going a few miles over the speed limit anyways so it works out well).

IMO, there are 3 main reasons why most roads are incapable of good dual signal progression.

#1.  Irregularly spaced traffic signals:  If the signal spacing between traffic signals is constantly changing along the corridor, then the cycle length that achieves perfect dual progression is constantly changing too.  If a set of lights has a perfect dual progression at a 90 second cycle, but the next set of lights has a perfect dual progression at 130 second cycle, what cycle do you time the corridor for?   Either you compromise and pick the cycle length that works best for the entire corridor or you break it up where some of the signals run different cycles altogether.

#2.  Signals are too closely spaced:  The most obvious example of this would be a two-way street in Manhattan like Broadway.   While the traffic signals are evenly spaced, they are only 260 feet apart.  Anybody driving Broadway intuitively knows that getting a string of green lights in both directions is impossible.  As speed of the corridor increase, the distance between traffic signals also must increase to be able to maintain dual progression.  The Woodward signals are spaced about 3000 feet apart and the speed limit is 45 mph.  Even along a Michigan left corridor that eliminate the need for left turn phases, the 90 second cycle just barely fits along Woodward.  If the signals were spaced any closer (or the speed limit was set any higher), the dual signal progression would be lost. 

#3.  Pedestrian crossings are too long: A side-street pedestrian crossing of 150 feet would need to run 52 seconds to adhere to the MUTCD and safely fit the pedestrians (there are literally dozens of major Orlando intersections with side-street crossings this long).  In general you want your main-street to run just as long, if not longer, than your side-street.  Assuming the main street also runs 52 seconds, the cycle length is up to 104 seconds.  But we haven’t accounted for left turn phases yet.  Add 20 seconds for both left turn phases and the cycle length is creeping up to 150 seconds.  Maybe a corridor in Orlando would achieve perfect dual progression at 100 seconds, but since the pedestrian crossing distances are so long it really can’t run that short of a cycle.  Technically you can use pushbuttons and only run the side-street the full 52 seconds when the pushbutton is pressed, but then the signal will get out of step with the surrounding signals every time a pushbutton is actuated.

I am so glad to see an in depth discussion of traffic control and engineering on this forum.  I wanted to expand a little on what tradephoric said in the above posts:

Conceptually, how can one program traffic signal progression on a 2-way street.

Let's take a simplified example.  We have Main Street with a 30 MPH speed (which of course is 1/2 mile per minute).  Signals are spaced every 1/4 mile.  Signals are set with a 60 second cycle length, meaning 30 seconds for our street and 30 seconds for cross-traffic.  [to keep the illustration simple, we will ignore all-red phases, the 30 seconds is the combined green and yellow time.]

You are at the corner of Main & A driving southbound.  The first 30 sec of the cycle (00-30) are dedicated to Main, and the second half (30-00) is dedicated to A.

The traffic on Main will reach B Street, 1/4 mile later, 30 seconds later.  So to ensure that all of this traffic will still see a green light, the portion of the signal dedicated to Main St traffic at the B street signal must be 30 seconds later than it was at the first signal.  I.e.  At the Main/B corner, (30-00) is dedicated to Main and (00-30) is dedicated to B.

The traffic on Main will reach C Street, 1/4 mile later, 30 seconds later.  So to ensure that all of this traffic will still see a green light, the portion of the signal dedicated to Main St traffic at the C street signal must be 30 seconds later than it was at the second signal.  I.e.  At the Main/C corner, (00-30) is dedicated to Main and (30-00) is dedicated to C.  Note, that this signal keeps the exact same timing as the Main/A signal.

So we have established that southbound Main will progress properly under the scenario.  Is it also true for northbound Main?  Yes.

You are at the corner of Main & C driving northbound.  The first 30 sec of the cycle (00-30) are dedicated to Main, and the second half (30-00) is dedicated to C.

The traffic on Main will reach B Street, 1/4 mile later, 30 seconds later.  So to ensure that all of this traffic will still see a green light, the portion of the signal dedicated to Main St traffic at the B street signal must be 30 seconds later than it was at the first signal.  I.e.  At the Main/B corner, (30-00) is dedicated to Main and (00-30) is dedicated to B.

The traffic on Main will reach A Street, 1/4 mile later, 30 seconds later.  So to ensure that all of this traffic will still see a green light, the portion of the signal dedicated to Main St traffic at the A street signal must be 30 seconds later than it was at the second signal.  I.e.  At the Main/A corner, (00-30) is dedicated to Main and (30-00) is dedicated to A.  Note, that this signal keeps the exact same timing as the Main/C signal.  Also note that the timing of the signal for both northbound and southbound platoons is simultaneous.  Southbound traffic leaving Main/B will reach Main/C at the same time that northbound traffic leaving Main/B reaches Main/A. 

So it is imperative that Main/A and Main/C have timing that is equivalent and that Main/B have timing that is opposite to the timing of Main/A and Main/C.  And this is accomplished by making sure that the cycle length of our signals are equal to the amount of time that it will take traffic to traverse the distance between two signals.  Since it takes 60 seconds to travel from A to C at 30 MPH, the cycle length should be 60 seconds to accomplish two-way signal progression.

This knowledge can be applied to derive the following useful formulas for setting the appropriate cycle length:

Cycle Length (sec)    =        7.2  X  Distance between signals (meters)  /  Speed (Km/hr)

Cycle Length (sec)    =        7200  X  Distance between signals (miles)  /  Speed (MPH)

Cycle Length (sec)    =        (15/11)  X  Distance between signals (feet)  /  Speed (MPH)

The last formula was used to plot the graph above, and you can see that it works.   For example, if the distance between signals is 2200 feet and the speed is 30 MPH, the cycle length is 100 seconds.  And on the graph, we see that the 30 MPH diagonal intersects the point that is (x=100, y=2200).

Many, many years ago there was decent progression in the Los Angeles area utilizing these formulas.  In many parts of the area, especially the San Fernando Valley and Orange County, the main streets were one mile apart.  At the half-mile points were important collectors and at the 1/4 and 3/4 mile points were the only other signalized intersections where local streets met the main streets.  If signals were kept simple (i.e. no provision for left turns) a citywide progression could be established with a 60 second cylce length at 30 MPH.  This was actaully achieved, one of many reasons why the city has historically been averse to installing left turn arrows.  Once the left turn arrows came, it became very difficult to fit in every phase within the 60 second cycle and progression was lost.

[GaryV]

I will agree that many major thoroughfares in Michigan have timing down pretty well (until traffic volumes increase so much that the roads can't handle it in general).  Many roads also have "smart" traffic lights that monitor the traffic flow and adjust the timing.

But in smaller towns and suburbs, sometimes they set the lights so you have to stop for most of them, as "calming".  Until you realize that if you go about 10 or 15 mph over the limit, you make the lights!

In Metro Detroit, I'm familiar with this phenomenon working on Woodward and on Telegraph, as I've seen videos for drivers along both streets cruising along.  Does it work on most other N/S streets?  The E/W streets?

Big Beaver / 16 Mile / Metro Parkway (especially the further east you are) is pretty good at it.

Some roads seem to have timing set per rush hour - better timing inbound in the morning, outbound in the afternoon.  But since much rush hour traffic is suburb to suburb, there's not as much of a concept of inbound/outbound for many areas.  I just know that if you're going the "wrong" way for that time of day, you probably will hit most of the lights red.

I remember years ago along US 31 on the east side of Holland there were even signs stating that the traffic light timing was set for 50 mph.  I haven't been there for several years.

[tradephoric]

To expand on the graph, it is showing the "critical"  resonant cycle length which is the highest cycle a corridor can run that will still achieve perfect 2-way signal progression.  So if the critical resonant cycle is 90 seconds but the corridor needs to run 120 seconds to fit pedestrians/left turn phases, perfect dual progression is impossible.  The generic formula used to plot this graph is the following...

Cycle Length (sec) = 2 X Travel Time

Where Travel Time = Distance / Speed.  Assuming distance in feet and speed in mph, the following equation is used....

Cycle Length (sec) = 2 X Distance between signals (feet) / (Speed (mph) X 5280 ft/mile / 3600 sec/hr

Rearranging the equation....

Cycle Length (sec) = (2 X 3600 sec/hr / 5280 ft/mile)  X Distance between signals (feet) / Speed (mph)

Simplifying it further, you get the same equation mrsman posted above...

Cycle Length (sec)  =  (15/11)  X  Distance between signals (feet)  /  Speed (mph)

[tradephoric]

^Now find a random suburban corridor in the country with a 45 mph speed limit.  A standard peak hour cycle length (especially if signals along the corridor have left-turn phases) is 120 seconds.  At 45 mph hour and a cycle length of 120 seconds, the signals would need to be spaced 3960 feet apart to achieve dual progression.   The corridor i choose was Colonial Drive in Orlando.  This roughly 2.5 mile section has several irregularly spaced traffic signals spaced much closer then 3960 feet apart.  You are just not going to get good signal progression down this road.







 

[tradephoric]

Extinct in most of Illinois (downtown Ottawa has it), it is fairly common in most of Michigan.

Examples:

The only traffic light the driver gets stopped at in the first 17 mile stretch (from Eureka to 18 Mile) is the SPUI at I-94.   Good signal progression may be a foreign concept for most cities, but when you drive down a corridor like Telegraph, progression destroying interchanges - like SPUI's - become painfully obvious.  Maybe it's not such a big deal to kill signal progression along some corridors, but Telegraph has traffic volumes comparable to many freeways... it's a major corridor in Detroit.  There are locations where SPUI's make sense, but I-94 / Telegraph isn't one of them (especially considering MDOT had so much ROW to work with when they were designing this interchange... the free-flowing interchange this SPUI replaced had a massive footprint). 

[tradephoric]

Here's another long drive down Telegraph where the driver doesn't get stopped at very many red lights (just one soft-stop).  This video extends to Dixie Highway which is also timed pretty well, but unlike Telegraph you don't get dual progression down Dixie.

https://www.youtube.com/watch?v=Wkf7iLSblZ0

[Brandon]

The only traffic light the driver gets stopped at in the first 17 mile stretch (from Eureka to 18 Mile) is the SPUI at I-94.   Good signal progression may be a foreign concept for most cities, but when you drive down a corridor like Telegraph, progression destroying interchanges - like SPUI's - become painfully obvious.  Maybe it's not such a big deal to kill signal progression along some corridors, but Telegraph has traffic volumes comparable to many freeways... it's a major corridor in Detroit.  There are locations where SPUI's make sense, but I-94 / Telegraph isn't one of them (especially considering MDOT had so much ROW to work with when they were designing this interchange... the free-flowing interchange this SPUI replaced had a massive footprint). 

Personally, I think the SPUI at I-94 and Telegraph would be a good candidate to replace with a DDI.  Instead of three cycles, you'd have two.  That would be more in place with the typical signal progression along Telegraph.

[Scott5114]

Sometimes other traffic concerns are more important than signal progression.

[Flint1979]

Michigan does it pretty well, as do the one-way avenues in New York City. It doesn't exist where I live.

And yes, the first light should be missed; it's green for people going straight, not people coming from a turn.

I don't see how Michigan does it that well it's about the same as anywhere else. You can get on roads here that'll make you stop at every light you encounter. Last night I was driving EB on M-46 coming into Saginaw and had to stop for the light at the entrance to Meijer at 11:30pm while the light at Miller Road was flashing, the light at River Road was flashing and the light at Midland Road (M-47) another state highway was flashing. While I was waiting for that light I thought to myself why in the hell is this light operating like this while all the other one's are flashing?

[tradephoric]

I don't see how Michigan does it that well it's about the same as anywhere else. You can get on roads here that'll make you stop at every light you encounter. Last night I was driving EB on M-46 coming into Saginaw and had to stop for the light at the entrance to Meijer at 11:30pm while the light at Miller Road was flashing, the light at River Road was flashing and the light at Midland Road (M-47) another state highway was flashing. While I was waiting for that light I thought to myself why in the hell is this light operating like this while all the other one's are flashing?

Meijer likely has a cost participation agreement with MDOT for the traffic light at their entrance and they have a say to how the light operates.  Since most Meijer's are open 24 hours a day, it's not surprising that the signal was operating at 11:30 PM.  But if all the other MDOT lights on M-46 were in flash, then there really isn't much signal progression to speak of (there needs to be more than one traffic signal to coordinate).  As you have alluded to, MDOT flashes many signals in the middle of the night.  Heck, they flash signals at freeway interchanges and major 4-phase intersections with left turn phases in the middle of the night.  But to me that's not a bad thing.  Personally, I don't understand your complaint.  Go to other states.  Many states cycle every signal 24/7... and you'll be getting stopped at more than just that Meijer light I promise you.

EDIT:  By the way, i presume all the traffic lights were flashing yellow for you on M-46, so you didn't have to stop? 

[jeffandnicole]

^Now find a random suburban corridor in the country with a 45 mph speed limit.  A standard peak hour cycle length (especially if signals along the corridor have left-turn phases) is 120 seconds.  At 45 mph hour and a cycle length of 120 seconds, the signals would need to be spaced 3960 feet apart to achieve dual progression.   The corridor i choose was Colonial Drive in Orlando.  This roughly 2.5 mile section has several irregularly spaced traffic signals spaced much closer then 3960 feet apart.  You are just not going to get good signal progression down this road.

With spacing such as this, I wonder if it's better just to try to have them all be green at one time, then red at one time.

The trouble will be that if the side roads all have varying traffic demands, and left turn channels have varying traffic demands, they all won't be able to turn green at the same time.  But, they should all be able to turn red at the same time, and then they cycle to the side street for whatever traffic they have.

You won't fly thru every intersection.  But if you stop at about 25% of them, that's better than needing to stop at half of them or greater!

[kphoger]

Sometimes other traffic concerns are more important than signal progression.

+1
Why do we think a signal is failing at its job just because it's hard to catch a green?

[tradephoric]

^Now find a random suburban corridor in the country with a 45 mph speed limit.  A standard peak hour cycle length (especially if signals along the corridor have left-turn phases) is 120 seconds.  At 45 mph hour and a cycle length of 120 seconds, the signals would need to be spaced 3960 feet apart to achieve dual progression.   The corridor i choose was Colonial Drive in Orlando.  This roughly 2.5 mile section has several irregularly spaced traffic signals spaced much closer then 3960 feet apart.  You are just not going to get good signal progression down this road.

With spacing such as this, I wonder if it's better just to try to have them all be green at one time, then red at one time.

The trouble will be that if the side roads all have varying traffic demands, and left turn channels have varying traffic demands, they all won't be able to turn green at the same time.  But, they should all be able to turn red at the same time, and then they cycle to the side street for whatever traffic they have.

You won't fly thru every intersection.  But if you stop at about 25% of them, that's better than needing to stop at half of them or greater!

This is a video that compares 3nd Avenue (a one-way street) and Park Avenue (a two-way street) in Manhattan.  The main point of the video is to show that the driver on the one-way street is limited to driving whatever speed the traffic lights are timed for.  Once they are near the start of the "˜green wave', the approaching red lights throttle back their speed even if they wish to drive faster.  OTOH, the simultaneous greens of Park Avenue incentivizes drivers to drive as fast as they can, since the faster they drive the more green lights they will make it through before the string of green lights turn back to red.

https://www.youtube.com/watch?v=6l7XbNK9M2s

Cities keep going back and forth on which is better... one-way or two-way streets.  There are some legit reasons why you may want to convert a one-way street back to a two-way street, but there is also a lot of disinformation out there.  For instance, proponents of two-way streets will often argue that since the average speed of traffic on a two-way street is lower than a one-way street, that two-way streets are safer for pedestrians.  But the average speed is based on travel time runs which account for both the time a vehicle is moving and the time they are stuck at a red light.  It's hard to kill a pedestrian when a vehicle is traveling 0 mph at a red light.

[TheArkansasRoadgeek]

It's hard to kill a pedestrian when a vehicle is traveling 0 mph at a red light.

Unless the car runs the red for any number of factors, to name a few: distrated driving, sudden medical affliction(s), intoxication, evading police, etc.

[kphoger]

It's hard to kill a pedestrian when a vehicle is traveling 0 mph at a red light.

Unless the car runs the red for any number of factors, to name a few: distrated driving, sudden medical affliction(s), intoxication, evading police, etc.

It's hard to run a red light when a vehicle is travelling 0 mph.   

[Flint1979]

I don't see how Michigan does it that well it's about the same as anywhere else. You can get on roads here that'll make you stop at every light you encounter. Last night I was driving EB on M-46 coming into Saginaw and had to stop for the light at the entrance to Meijer at 11:30pm while the light at Miller Road was flashing, the light at River Road was flashing and the light at Midland Road (M-47) another state highway was flashing. While I was waiting for that light I thought to myself why in the hell is this light operating like this while all the other one's are flashing?

Meijer likely has a cost participation agreement with MDOT for the traffic light at their entrance and they have a say to how the light operates.  Since most Meijer's are open 24 hours a day, it's not surprising that the signal was operating at 11:30 PM.  But if all the other MDOT lights on M-46 were in flash, then there really isn't much signal progression to speak of (there needs to be more than one traffic signal to coordinate).  As you have alluded to, MDOT flashes many signals in the middle of the night.  Heck, they flash signals at freeway interchanges and major 4-phase intersections with left turn phases in the middle of the night.  But to me that's not a bad thing.  Personally, I don't understand your complaint.  Go to other states.  Many states cycle every signal 24/7... and you'll be getting stopped at more than just that Meijer light I promise you.

EDIT:  By the way, i presume all the traffic lights were flashing yellow for you on M-46, so you didn't have to stop?

I regularly report gas prices along Gratiot (M-46) on Gasbuddy so I'm out that way quite a bit. I never really noticed if it flashes later into the night. The entrance is kind of strange because it's really a side street that goes on the side of Meijer next to the gas station. I thought that this light flashed at some point of the night, possibly it starts flashing at midnight.

Yes the rest of them were flashing yellow, I forget if the one at Graham Road (M-52) was flashing but it seems like it was, I was on M-46 until Center which was also flashing, Miller, River, Midland, Frost and Center all were.

[tradephoric]

It's hard to kill a pedestrian when a vehicle is traveling 0 mph at a red light.

Unless the car runs the red for any number of factors, to name a few: distrated driving, sudden medical affliction(s), intoxication, evading police, etc.

It's hard to run a red light when a vehicle is travelling 0 mph.   

Exactly!  But to ArkansasRoadGeek's point there will always be isolated incidents where distracted drivers completely blow through a red light.  But what distracted driver is more likely to blow through a red light in that video?  The one-way driver who gets an endless string of green lights as long as they keep up with the flow of traffic, or the two-way driver who is guaranteed to encounter a red light every minute or so (since Park Avenue is timed for simultaneous greens and a red light is inevitably coming).  The one-way driver can be playing Candy Crush on their phone, glancing up every 10 seconds to the road, and they still may have trouble blowing through a red light.  It's just hard to "catch"  a red light on a properly timed one-way street.  OTOH, it's really easy to "catch"  a red light on a two way road where traffic signals are spaced closely together.

[kphoger]

there will always be isolated incidents where distracted drivers completely blow through a red light roundabout. 

You did know that was coming, right?   

[tradephoric]

there will always be isolated incidents where distracted drivers completely blow through a red light roundabout. 

You did know that was coming, right?   

LOL I was expecting that!  To improve intersection safety, retractable bollards should extend up from the roadway when a signal turns red.  That way if anyone blows through a red light, they will crash into the sturdy posts before they have a chance of t-boning anybody in the intersection.  No more need for roundabouts!  Here is a crash test to simulate how safe an intersection would become when someone tries to blow through a red light!

https://www.youtube.com/watch?v=9FfUlojxiqE

[kphoger]

there will always be isolated incidents where distracted drivers completely blow through a red light roundabout. 

You did know that was coming, right?   

LOL I was expecting that!  To improve intersection safety, retractable bollards should extend up from the roadway when a signal turns red.  That way if anyone blows through a red light, they will crash into the sturdy posts before they have a chance of t-boning anybody in the intersection.  No more need for roundabouts!  Here is a crash test to simulate how safe an intersection would become when someone tries to blow through a red light!

https://www.youtube.com/watch?v=9FfUlojxiqE

What about emergency response needing to drive through red lights?

[roadfro]

LOL I was expecting that!  To improve intersection safety, retractable bollards should extend up from the roadway when a signal turns red.  That way if anyone blows through a red light, they will crash into the sturdy posts before they have a chance of t-boning anybody in the intersection.  No more need for roundabouts!  Here is a crash test to simulate how safe an intersection would become when someone tries to blow through a red light!

I'm hoping this wasn't a serious recommendation...

[Brandon]

LOL I was expecting that!  To improve intersection safety, retractable bollards should extend up from the roadway when a signal turns red.  That way if anyone blows through a red light, they will crash into the sturdy posts before they have a chance of t-boning anybody in the intersection.  No more need for roundabouts!  Here is a crash test to simulate how safe an intersection would become when someone tries to blow through a red light!

I'm hoping this wasn't a serious recommendation...

If it were, I can just imagine the carnage cause by the bollards in Chicago where red light running is a popular sport.