Traffic Signal Timing and Phasing

This is Part Three in a series on traffic signal controllers. Part One covered an overview of a hardware, and Part Two continued with general issues of why they don’t always have features we’d like. As of note about terminology, I’ve generally used the term “vehicle” and “vehicle operations” to match engineering terminology, despite that fact that it is ultimately the human motorist whose time is wasted with poor vehicle operations, and that a bicycle is a vehicle, but it can be on the sidewalk or sidewalk / trail rather than the street, and the rider is affected differently than motorized vehicles. This part focuses on timing and phasing issues.

The Case for Long Cycle Times- Intersection Efficiency 

During each cycle you have some time where no one is utilizing the intersection. For simplicity let’s say the yellow and all red times are when no one can use the intersection.  (Although it’s well known motorists will “run” the yellow, and they can’t start immediately on green, in the real world these considerations usually exactly cancel each other out.)

Let’s take two extremes. 20 second Main Street green, and 7 second Side Street green, three second yellows, and  two second all reds. That means for that cycle only 73% of the time is green.

graph 2a

Now how about an 8 minute Main Street green (as actually implemented for US 10 in Royalton on Friday summer evenings) and say a 30 second Side Street green. Here 98% of the capacity is green.

long cycle times

Obviously there’s disadvantages to longer cycle times. Pedestrians have to wait longer in the cold if they want to cross the street that’s green. Motorists start to fume and may turn around and cut through neighborhood streets. The engineers’ phones start to ring with complaints. Like so much else, setting cycle times is a compromise based on judgement and trial and error.

Protected-only turn phases (you can only turn on a green arrow) have a pretty negative impact to vehicle operations. With protected phases both in directions, you’ve just doubled the yellow and red times per cycle, plus some of the turning traffic that time is devoted to may have been able to utilize gaps in the main phase had they been allowed to.

The Capacity Killer

The Capacity Killer

The Case for Short Cycle Times: Progression 

But why can’t lights be timed so you hit a series of greens? They try to (sometimes). And have been since almost the beginning of traffic signals. And computer modeling is making it a lot easier. But the suburbs, with their irregular traffic, irregular spacing between traffic signals, and frequent use of turn phases make it extremely challenging. One of the reasons you’re seeing many more where the turn arrow is at the end of the phase in one direction is that it was determined that is the most efficient way to coordinate (and the flashing yellow arrow makes it safe to do so with protected/permissive by eliminating the yellow trap).

Another problem is that although long phases are more efficient at moving traffic through an isolated intersection; short phases are better for progression. For progression on a two-way street you want to have a cycle take twice the time it takes a motorist to travel from one signal to another. Here’s a video of a NYC driver catching a green wave for 125 blocks. This is the ideal situation: late at night with little traffic on a one way.

Fixed time lights can actually work quite well in dense areas. A while ago NYC disconnected many sensors and reverted to fixed time (and left the pedestrian buttons abandoned, leading to the myth that they “don’t do anything”.

Portland and San Francisco, with their noted anti-car, pro-bicycle/pedestrian culture, have timed signals to create a green wave for bicycles.

In downtown areas with signals every direction every block, it may be better just to have a string of lights turn green at the same time. This is easier to implement and if traffic is gridlocked it all starts moving at the same time.

Pedestrian Times 

Moving into pedestrian times first we need to define some terms. We have the walk interval (the time the “Walk” sign is illuminated), the pedestrian change interval, (when “Don’t Walk is flashing), and the buffer interval, (when the “Don’t Walk” sign is illuminated steady). The clearance time is the combination of the change interval + buffer interval. Here’s a graphic illustration, Minnesota uses the first option, where the pedestrian buffer interval starts with the vehicle change interval (the yellow light). Overlapping the buffer with the red clearance interval, like the 2nd, 3rd, and 4th options can reduce overall cycle times or increase the walk interval, but is problematic to program and reduces the time for pedestrians to scramble out of the intersection after the sign goes steady and before vehicles are released.


Pedestrian times are set with the following parameters:

1) The walk interval must be at least 7 seconds, unless it’s determined that 4 seconds is adequate in a specific location.

2) The clearance time is the distance from curb to curb in feet divided by 3.5 feet / second walking speed.

3) As mentioned above, in Minnesota, there is the two second red clearance interval after the clearance time.

4) There is another calculation that is made: the time of the entire phase must equal the distance from the near push-button to the far curb divided by 3.0 feet / second. In practice this doesn’t become the limiting factor until road widths become extremely large, in the order of 100 feet or more, so we will ignore it for a while

5) 4.0 feet / second walking speed was formerly the national standard, but is now only allowed if there is a provision for a pedestrian requesting extra time at the button, or pedestrian sensors in use. I am not aware of these features being implemented in Minnesota.


Refuge islands  count as far as setting time to the far curb; in practice the time is usually more generous than the minimum, since the goal isn’t to make every pedestrian wait in the middle, just reduce the times so the slowest possible pedestrian doesn’t impact vehicle operations too much.

Pedestrian Recall- Why or Why not.

Sometimes the “Walk” sign goes on every cycle (ped recall), and sometimes not. Generally speaking:

1) In a fixed time intersection, there is no provision for vehicle sensors or pedestrian push-buttons, so ped recall is enabled all the time in all directions.

2) In a semi-actuated intersection, some directions have sensors and some don’t, so ped recall may be enabled some direction but not others. In the past the presence or lack of pedestrian buttons was a good indication of whether an intersection was actuated or not, but with the advent of accessible pedestrian signals they all have buttons.

3) For fully actuated intersections, ped recall may or may not be enabled if it can be without degrading vehicle operations. Sometimes the limiting factor in how short an intersection phase can be is the pedestrian phase, sometimes the vehicle phase. Lets look at two scenarios.

Scenario 1: 56th Street at Penn Ave , 56th is 35 feet wide, Penn is 40 feet wide. 40 second minimum vehicle phase on Penn, 15 seconds on 56th. The minimum pedestrian phase is  21 seconds (7+40/3.5 + 2) in theory across Penn, 19 seconds on 56th (but is 25/22  seconds as programmed- they made the change interval longer than what is required) Vehicle phase is the limiting factor across 56th, Ped phase is across Penn. So ped recall is programmed in the former but not the later.


The only impact to vehicles is if a vehicle arrives on 56th and there is no traffic on Penn after the minimum green time, they may have to wait through Penn’s change interval before getting their green. This is mitigated somewhat by having the ped signals repeatedly count down and then go back to walk, at the cost of making a pedestrian wait if they don’t arrive during one of the walk intervals.

Scenario 2: American Blvd at Grand Ave (better known as  Home Depot Driveway) Six lane wide suburban-style road about 90 feet wide,  15 second vehicle phase for Grand Ave.  If a pedestrian phase is present, the minimum time is 35 seconds (7 + 90/3.5 + 2) both in theory and practice, so a pedestrian phase impacts vehicle operations pretty severely. It’s such a problem that engineers try to deal with it by either by adding a refuge island in the center (as has been discussed as a solution for various places on American) so a slow pedestrian has to use two cycles to cross. Or by fudging on the timing so the 3.5 feet / second rule only gets to the center of the last lane (the theory no doubt being the driver in the last lane is going to look before starting…).


There is no ped recall on any crossing at Grand and American, but with the 70 second phase for American Blvd. an engineer could enable it to cross Grand with essentially zero impact to vehicle operations. Why don’t they? A blanket policy not too? Didn’t think of it? The way it’s always been done? I don’t know. (The city of Bloomington has been less than friendly and responsive with my dealings with them so I didn’t even attempt to get an answer). Engineers (and from what I perceive of  the general public) just don’t see the act of pushing a button either as especially difficult nor the supreme ideological insult.

 Exclusive Pedestrian Phases (X-Ped)

Ped Recall may or may not have an impact on vehicle operations, but exclusive pedestrian phases almost always have an extremely severe impact.  Rather than sharing the intersection, vehicles and pedestrians use the intersection sequentially, the most inefficient way possible, and a ban on right turn on red is essentially required.  The problem is compounded by the recent reduction of assumed pedestrian walking speed from 4.0 to 3.5 feet a second. The vehicle delays are so extreme that Denver, not exactly a bastion of conservatism, removed the last of theirs fearing gridlock with the reduction of walking speed and the coming of light rail.

So let’s chart our intersections “as is”, with ped recall on all phases, and with an exclusive pedestrian phase. (To keep things simple I’ve used the theoretical 3.5 ft/s walking time even though they are more generous than that on Penn.) Wasted Green Time is when the vehicle signal has to be green because of the pedestrian phase, but no vehicles are there. And we’ve already calculated concurrent phasing, so let’s calculate exclusive phasing. Penn Ave S. at 56th is 65 feet diagonally, 7 + 65/3.5 = 26 seconds. American Blvd at Grand Ave is 11o feet diagonally from curb to curb, 120 feet from button to far curb. This is getting pretty wide, so we better check both calculations. 7+ 110/3.5 = 40 seconds, 120/3 = 40 seconds. So interestingly this is exactly where the two calculations meet.

graph 1

graph 2b

Chart ZZZ

graph 4

graph 5a

graph 6

Obviously this has a big impact to vehicle operations. But what if we decide that motorists can sit and stew because all we care about is pedestrians? Well, it’s not so clear then either. There are drawbacks to X-ped to pedestrians too. It increases their average wait time. (One intersection that was modeled increased it from 34.7 seconds to 49.5 seconds- LOS D to LOS E.) The tendency is to get impatient and jaywalk against the “Don’t Walk” sign, where motorists see they have the right-of-way and thus are not looking for pedestrians. An Israeli study showed  that X-Ped is safer as far as vehicle vs pedestrian crashes at heavy vehicle volumes (18%), but more dangerous at lower vehicle volumes (-8%). Like most everything else in engineering it’s a trade-off between safety and efficiency.

Minnesota's busiest intersection- MN 252 at 66th Ave N

Don’t expect an exclusive pedestrian phase any time soon at Minnesots’s busiest intersection

The next article will continue with commonly asked questions about signals.






About Monte Castleman

Monte is a long time "roadgeek" who lives in Bloomington. He's interested in all aspects of roads and design, but particularly traffic signals, major bridges, and lighting. He works as an insurance adjuster, and likes to collect maps and traffic signals, travel, recreational bicycling, and visiting amusement parks.

23 thoughts on “Traffic Signal Timing and Phasing

  1. Sean Hayford OlearySean Hayford Oleary

    “Portland and San Francisco, with their noted anti-car, pro-bicycle/pedestrian culture, have timed signals to create a green wave for bicycles.”

    So, since Minneapolis times one-way pairs like 26/28th to create a green wave for car speeds, should we say Minneapolis has a “noted pro-car, anti-bicycle/pedestrian culture”? This is a great article, but that seems like a cheap shot.

    The ped recall details are good. The configuration like 56th & Penn seems to be the most common for new signals in Minneapolis, St. Paul, as well as other jurisdictions like Dakota County.

    Why Hennepin County — which I would think considers itself more progressive and ped-friendly than Dakota County — can’t at least match Dakota’s pedestrian consideration with their signals is ridiculous. Your assessment that enabling ped recall for pedestrians walking along the major road is almost always correct. I have been told that Hennepin County will consider changing the programming on certain signals, if they’re in a particularly ped-oriented area, but I’ve yet to test it. (Hoping to see some ped recall along the signals that remain on 66th.) MnDOT operates like this. I’ve noted in both Wadena and Lindström, new MnDOT signals have ped recall for the dominant roadway.

  2. Monte Castleman Post author

    Urban transportation is usually a zero or negative sum game, so you can’t favor one mode without being against the other since you know it will cause impacts. San Francisco and Portland have chosen to favor nonmotorized traffic in a lot more ways than green waves for bicyclists instead of vehicles. So you could characterize 26th as pro-car, anti-bicycle, perhaps even this region as a whole, but based on other factors, not just that. I’ve driven in San Francisco and wished I hadn’t, but that’s their city and I’m the guest and I don’t have to live there.

    1. Sean Hayford OlearySean Hayford Oleary

      > “Urban transportation is usually a zero or negative sum game, so you can’t favor one mode without being against the other”

      But I think your ped recall example proves that you can make one mode better (walking) without hurting another mode (driving). Not everything is like that, but there are plenty of examples:

      1. Roundabouts reduce both vehicular delay and ped delay (since they always have right-of-way)

      2. Removing free rights can reduce rear-end accidents while improving ped safety and experience

      3. 4-to-3 conversions at the right volume can reduce delay and make the roadway more predictable for motorists while improving bike and ped experience.

      4. Increasing lighting pretty much benefits everyone (except perhaps adjacent residents, if the lighting level is too high)

      In Bloomington, of course, improving “pedestrian safety” seems to involve numerous benefits to the motorist — like improving “safety” by forcing pedestrians to cross at a grade-separated skyway on Killebrew Drive, which just so happens to reduce delay for motorists at lights. Or how the Lindau Lane “complete street” project allowed the city to build a near freeway stub to 24th Ave.

    2. Alex CecchiniAlex Cecchini

      Maybe it’s just semantics, but I’m curious what urban transportation investments would be negative-sum, either in experience, safety, or even total spending.

      I’d also challenge the notion that either of Portland or San Francisco (maybe anywhere in North America) favor non-motorized traffic in a broad sense. I would bet that most (at least a plurality) of the public right of way in both cities is still reserved for motorized traffic. Very few streets have bus-only lanes. Some have dedicated bike space. As a litmus test, major streets through SF dedicate more space to parking cars than bikes or buses. I think design and control elements that favor (or slightly prioritize) non-motorized traffic are relatively rare in both cities. The overall DOT philosophy still sees the transportation network within the city as how to efficiently move cars except in places where ped or bike or transit volumes are too high to justify anything else.

      Anecdotally, I didn’t have an issue driving in San Francisco. Came across the GGB and had very little trouble getting to my hotel. That same vacation included driving on a freeway right through the heart of Portland. So things weren’t so tough on me as a motorist, IMO.

      1. Monte Castleman Post author

        An example of what I’d call a negative sum investment would be enabling ped recall across American. Very small objective for pedestrians (not having to push a button for the occasional pedestrian, I didn’t see a single one the entire time I was there; extreme downside for vehicles (tripling the green interval for the side street when no vehicles need it).

        I guess the actual driving in San Francisco isn’t too bad, it was trying to find a parking space near Lombard Street that was open and usable for someone not skilled in urban parking techniques. It took about a half hour to drive from my hotel in Walnut Creek to San Francisco, and about that repeatedly circling around to find parking.

        1. Sean Hayford OlearySean Hayford Oleary

          Of course, that gets tricky, doesn’t it? I’m not sure the lack of ped recall makes a significant difference to ped attractiveness, but I would bet you’d see a lot more pedestrians if the City had planted heavy, consistent boulevard trees along the street. That would be “negative” for cars, potentially, since they wouldn’t have as graceful a clear zone. But it would likely attract more pedestrians to the corridor, and certainly vastly improve the experience for those using it.

          When the starting point is extreme auto orientation (and the mode share breakdown is a result of that), it’s hard to make judgments about how many people experience a benefit vs. cost when a given change could make a difference to that mode share.

        2. Rosa

          So I have an appointment on American every Thursday, very near the MOA, and since we have the car out anyway I try to layer other car errands on. I see a surprising number of pedestrians, for how unwelcoming that road is (no shade, among other things). The difference between that area and Grand might just be that the infrastructure is worse (or maybe there’s a city bus line that doesn’t go all the way West to Grand? I don’t even know the bus schedules down there)

          That said, my appointment starts at 3 pm and I see a LOT of pedestrians on that whole strip from Cedar/77 over to the little mall (what is that, Southpoint?). There are apartment buildings just south of American, and school buses stop there for families that live in them. There are often people walking to the WalMart and I assume the restaurants over toward there.

          I keep thinking we’re going to take the light rail and then bike over from the Mall, but it’s pretty daunting – I’d totally do it on my own but with the kid, it’s actively frightening.

          But I do wonder what the real cost of the beg button for pedestrians is. One is that you’re right, it just leads to jaywalking – it’s pretty easy to know when the light says “don’t walk” when the cross traffic has all red lights, so it’s pointless to follow it. But also is there even a situation that’s similar for drivers, where if (for instance) you want to turn right or left but you weren’t in the turn lane before the forward traffic got its green, you’re supposed to just wait an entire cycle? Because that’s what happens with the walk lights. You show up, it’s clearly a time when you could walk, but you can’t get a walk light without waiting forever (and even when you get it, cars turn through it, so it’s functionally exactly the same.)

          It’s not a big deal at the intersections you mentioned but that habit of generally walking against the light can be actively dangerous at a complicated intersection like the one at Lake & Hiawatha, where it’s harder to know the status of all the various car lanes (and where there’s very little point in waiting for the walk light because it’s barely long enough to get across on a bike, much less on foot with little kids.)

          1. Sean Hayford OlearySean Hayford Oleary

            Route 5 is the highest-frequency and busiest route, that runs on American between Portland Ave and 24th Ave. Route 542 (less frequent) also runs on almost all of American. Walmart is a major destination on the 5, for people from south Minneapolis as well.

            The most notable gap is from 12th to Chicago, with no sidewalk on the south side but a well-trod cowpath. No idea why it’s missing altogether.

            But as we seem to agree, the sidewalk that’s there (although vastly better than having no sidewalk) is positively miserable. This section seems particularly grim, baking between 5-lane stroad and parking lot, nothing to look at but fast cars and an unadorned cinder block retaining wall. Compare that to 77th Street across the freeway. Still a high-speed stroad, but greenery and shade make it infinitely more humane and comfortable to walk along.

          2. Monte Castleman Post author

            Protected only turns are still pretty standard in the suburbs, so if you arrive at the left turn lane after through traffic has it’s green you have to wait through the entire cycle, just like a pedestrian arriving after the walk interval.

            The problem you mention isn’t pedestrian pushbuttons or no. All they change is if the walk interval goes on at the beginning of a phase or not. The intersection I mentioned at American it will reservice (provide a walk interval) if you arrive late and push the button and there’s time in the vehicle phase to provide one. They don’t do anything to change the clearance time.

            As a thought expeiremant, now that we have countdowns what if we dispensed with the long clearance time, allowed pedestrians to legally enter the intersection any time during the vehicle green if they thought they could make it across, and having the countdown start at the beginning of the green.. A jogger arriving at 10, or a bicyclist on the sidewalk arriving at 5, could make it across, but would it increase accidents with pedestrians misjudging if they can make it across and motorists not seeing them before starting out? I don’t know.

            1. Sean Hayford OlearySean Hayford Oleary

              I would be supportive of that, as long as pedestrians had the opportunity to push and wait for a new cycle with additional time. The way our ped laws are written today, it is illegal to enter the intersection after the pedestrian clearance interval has begun. For 90% of sidewalk users at 90% of lights, this is ridiculous. Say Hiawatha and 28th — I believe the clearance interval is around 45 seconds. I can make it across on 5 on bike. Legally I should stop with the hand flashing 45, wait for that to clear out, wait for the whole cycle for Hiawatha, then wait for a fresh walk signal.

              Half the benefit of having countdown timers is allowing people of different abilities to make appropriate judgments. Time for our law to catch up with the equipment.

              1. Monte Castleman Post author

                If you think back to the early days, some lights would go directly between red and green with no yellow or all-red. This is when pedestrian signals, and the concept of pedestrian clearance time got started (originally it was a “blind clearance” with only a walk light). Probably the best with the limited information such primitive signals provided, but it may be time to rethink the paradigm with better technology.

                If we wanted to we could lengthen the buffer interval to some arbitrary figure too if we thought it wasn’t safe to allow sidewalk traffic to enter with only five seconds left, countdown or no, while still substantially increasing the non-motororized LOS over the present long clearance time.

                It’s also notable the first countdowns began during the walk interval. Some modules still have DIP switches to select this even though it’s no longer allowed.

              2. Rosa

                and at that same intersection, the interval is so short there are quite a few pedestrians who can’t make it anyway.

            2. Rosa

              wait, there are a lot of places where you’re only allowed to turn on green arrows? Or is it just that, functionally, you never get to turn because there are no gaps in the oncoming car traffic and cars continue to drive right through the amber and some of the next red light? Because that’s pretty typical in my neighborhood.

              I would love to see the automatic walk signal/countdown thing studied – pedestrians have to be so careful anyway I can’t imagine there would be that much of a change, but it would be nice to see actual behavior.

              1. Monte Castleman Post author

                There really are lots and lots of places where you can only turn on a green arrow. Rare in the cities but extremely common on major streets in the suburbs.

  3. Kevin

    That taxicab video was interesting. Made me remember one of my 3am cab rides home from Midtown to Brooklyn back in the day, just shooting through green lights full speed ahead all the way from 42nd Street all the way to the Manhattan bridge. Experienced cabbies know how to fly their fares to their destination as quick as possible, which always struck me as an art back in my NYC days.

  4. Bill LindekeBill Lindeke

    This is a great piece, Monte. Thanks for trying to explain this rather difficult subject. It’s so weird to see ped signal timings in chart form.

    A friend of mine once gave me a lecture about ped timing in crosswalks and I was fascinated. I’ve been wondering about them ever since.

    Are there different kinds of tradeoffs in more urban places (w/ narrower streets) vs. more suburban places? Why do some cities have quick phases (e.g. L.A.) while others seem to have very long ones?

    1. Monte Castleman Post author

      Maybe I shouldn’t have commented on what I see the culture of other cities as (and I haven’t even been to Portland), but I’m happy that people are moving beyond me being me and finding the actual content interesting.

      I think the basic trade-off remains true regardless of street width- longer cycle times maximum the efficiency of a intersection in isolation and a pedestrian phases has less effect on the vehicle phase, shorter phases are necessary for progression and make pedestrians wait less. There’s no federal guidelines for timing (apart from absolute minimum times) so which direction to go depends a lot on the philosophy on the engineers of the region.

      There’s a comment on another article that there’s some 2-3 second walk intervals around. To put it bluntly those don’t meet standards and should be complained about to the local agency. There’s probably a lot around too that haven’t been retimed from 4.0 ft/s. And there’s the acknowledged but unsanctioned shortening of the clearance time across wider roads I mentioned, in my opinion this could expose an agency to liability if there’s a crash.

      But provided everyone can make it off the curb, extending walk intervals doesn’t necessary benefit pedestrians. If it can be done without affecting the vehicle phase it can, should, and often is done. But if it so long as to extend the vehicle phase, then it’s offset by pedestrians the other direction having to wait longer.

      1. Joseph TottenJoseph Totten

        I’d like to hear your comments on the short vs long cycle length as it relates to LOS if you would.

        Typically LOS is for minimizing average delay, but like on American, this can be really strange, long cycle times. (Assuming no clock for different times of day…) Sometimes short cycle switches are preferable to very long cycles lengths, even if that means the queue won’t entirely clear (essentially, if you’re going to fail, stop trying to force it not to fail and let it not mess up the rest of the system).

        Sorry if this is unclear at all, please ask for more information.

        1. Monte Castleman Post author

          I think the basic rules never change that, in isolation, the most efficient way to run an intersection is to have long cycle time because the red and yellow times are reduced as a percentage of the cycle; in a system there’s other considerations with respect to the overall system.

          I’m not an engineer, so I don’t want to get beyond that. I play with things that light up after my day job; not get paid sit at a desk and do LOS calculations and progression schemes. I looked at the LOS formulas for vehicles and pedestrians and decided not to get in over my head figuring out hypothetical LOS for various cycle lengths.

  5. Nathanael

    So, interesting moves. My city (Ithaca NY) just replaced the lights at a particularly pernicious intersection as part of a larger project (the poles had to be moved so why not replace the lights.)

    Anyway, the new arrangement is as follows.

    — There are pedestrian buttons… but all they do is activate the audible announcements. The lights are actually on a completely fixed timing.
    — The pedestrian cycles start *before* the cars are given a green light, by two or three seconds. This deters right-turning cars from running over pedestrians, because the pedestrians can be well into the middle of the road, past the car’s ‘blind spot’, before the right-turner sees a green light. This is important because the predominant traffic flow is actually a right turn (it’s a T-intersection.)

    1. Nathanael

      To be clear on this, the ped signal goes “walk” at the beginning of the “all-red” phase for cars. So the moment the cars get a red light rather than a yellow light, you can walk in front of them.

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