With Yellow Lights, Timing Is Everything

[cpzilliacus]

Wall Street Journal (paywalled - if you cannot see the full article, PM me with an e-mail address and I will send a link to it): With Yellow Lights, Timing Is Everything - 'Just right' has proven elusive, but traffic engineers may help change that with new guidelines

The yellow light is the Goldilocks of the traffic signal. If it is too short, drivers are likely to run a red light by mistake. If it is too long, they may intentionally risk it to zoom through an intersection.

"It's a lot like medicine,"  said Richard Retting, a veteran transportation engineer. "There needs to be a dose that's appropriate."

But traffic engineers in the U.S. have never agreed on how to time yellow lights so the interval between green and red is just right. Federal guidelines specify only that yellow-light intervals must fall between 3 and 6 seconds.

The last time the Institute of Transportation Engineers tried to come up with a formula, it failed to muster enough support among its 15,000-plus members to adopt a proposed recommendation.

That was 30 years ago.

"It's just a matter of validating appropriate input values,"  Mr. Gates said. "If the formula suggests you need 4.5 seconds, and if you provide drivers with 3 seconds, you're putting drivers into a case where they either have to stop abruptly or proceed through and run the risk of running the red light."

That was what was happening in Chicago last year–apparently by design.

According to an investigation by the Chicago Tribune, the city began issuing red-light tickets when the yellow interval dipped to 2.9 seconds, giving drivers slightly less time to react than normal. Previously the city didn't issue tickets if the yellow interval lasted less than 3 seconds. The difference resulted in an additional 77,000 red-light camera tickets worth $7.7 million.

Three seconds is still a close shave. If the city applied the new proposed formula, flat intersections operating at the standard 30 mph speed limit would have a yellow interval of 3.71 seconds.

[jakeroot]

The new formula specifically recommends using vehicle speed rather than a posted speed limit, but in reality, few agencies collect that data, so the formula also offers an approximation.

Aren't speed limits themselves based on average speeds (i.e. the 85th percentile)? How are speed limits set if agencies don't know how fast drivers are going? (outside of downtown areas, where a limit above 30 is ridiculous).

[Big John]

^^ Actually speed limits are supposed to be the 85th percentile speed of the road.   But all too often a politician will set it unreasonably low and write it into law.

And yellow light intervals are also supposed to be based on the prevailing speed of the road.

[jakeroot]

^^ Actually speed limits are supposed to be the 85th percentile speed of the road.   But all too often a politician will set it unreasonably low and write it into law.

And yellow light intervals are also supposed to be based on the prevailing speed of the road.

Sorry, yes, I meant 85th percentile. I updated my post to reflect the discrepancy.

[Duke87]

Aren't speed limits themselves based on average speeds (i.e. the 85th percentile)?

Have you driven around much?

Speed limits are often established directly or indirectly by statute. Some states have laws which say "the speed limit on this type of road shall be this number unless the DOT does a study and determines it should be something else". These are the states where when they change their law to raise the maximum speed limit the signs change all over the place the instant it goes into effect. Every state has a law saying something to the effect of "the speed limit on this type of road shall not exceed this number", which is often the cause of underposted limits.

Then you have cases where NIMBYs whine about people driving too fast down a road, and politicians respond by decreeing the speed limit shall be lowered and cops shall be sent out to strictly enforce it.


As for the availability of percentile speed data, while this is available for most major roads at a series of traffic counting points, and speed limits could be based on it if politics didn't meddle... the speed of traffic at the point where it is counted is not necessarily the same as the speed of traffic approaching a particular intersection, and the latter is what governs signal design.  Consider a windy road that has a long straight downhill leading up to a signal afterward. The average speed on the road measured amongst its curvy section is going to be lower than the speed down the hill.

Also, while this data may exist for major roads, it's likely not going to exist for every city street that has a traffic signal on it. Even for signals on major roads, the speed data is not going to exist for all of the signalized side streets.

[jakeroot]

Aren't speed limits themselves based on average speeds (i.e. the 85th percentile)?

Have you driven around much?

<clipped>

Hmm. Very good points.

Instead of refuting them, I'm going to simply just abandon this thread.

[roadfro]

And yellow light intervals are also supposed to be based on the prevailing speed of the road.

The ITE formula for the yellow clearance interval includes 85th percentile speed as one of the variables (in the absence of true speed study data, the speed limit is commonly used for this value instead).

ITE also has a formula for the "all-red" interval, which uses a 15th percentile speed. (Not all agencies include the all-red interval before cross traffic receives a green.)

[riiga]

Sounds like quite a troublesome situation, 3 seconds is way to short an interval on a 50 km/h (30 mph) road. The proposed formula's 3.71 seconds seems much more reasonable.

I'd be interesting to know what intervals other countries use. Sweden requires an interval of 4 seconds for roads where the speed is lower than 60 km/h (40 mph) and 5 seconds where the speed is 60 km/h or higher. However, if there are good reasons and it can be done without impacting traffic safety the interval may be lowered by up to 2 seconds, unless there is a fair amount of bike traffic in which case the interval may only be lowered 1 second. Also, on roads with a speed limit of higher than 70 km/h (45 mph) traffic signals are prohibited.

[Brian556]

The thing I don't get is why do they routinely have too-short yellows at intersections that do not have cameras as a motivation to do so?

[Zeffy]

The worst thing is having to run a potentially red light because the guy behind you is on your ass and he sure as hell won't stop in time if you do.

[Duke87]

Sweden requires ... Also, on roads with a speed limit of higher than 70 km/h (45 mph) traffic signals are prohibited.

Now this I find intriguing. In the US, there are plenty of roads with speed limits higher than that where a traffic signal may be encountered. Signals on roads posted at 65 definitely exist and I would be surprised if someone found one on a road posted at 70 (although I can't think of an example offhand).

Does this policy mean that a high speed road will be underposted at 70 km/hr if a traffic signal is present? Or does it mean that the relevant authorities will go to great lengths to avoid this situation?

[UCFKnights]

Sweden requires ... Also, on roads with a speed limit of higher than 70 km/h (45 mph) traffic signals are prohibited.

Now this I find intriguing. In the US, there are plenty of roads with speed limits higher than that where a traffic signal may be encountered. Signals on roads posted at 65 definitely exist and I would be surprised if someone found one on a road posted at 70 (although I can't think of an example offhand).

Does this policy mean that a high speed road will be underposted at 70 km/hr if a traffic signal is present? Or does it mean that the relevant authorities will go to great lengths to avoid this situation?

I don't think Florida will put signals on roads set at 65mph (Florida's maximum for non-highways). When they need a signal, they post a 60mph (or sometimes 55mph in some counties) sign about 1000ft before the intersection, and then raise it back up 1000ft after it.

[riiga]

Does this policy mean that a high speed road will be underposted at 70 km/hr if a traffic signal is present? Or does it mean that the relevant authorities will go to great lengths to avoid this situation?

It means that speed will be lowered to 70 km/h in time before encountering the signal, but also that we tend to avoid this situation by building roundabouts, non-signalized intersections or simple grade-separated interchanges instead, especially in rural or semi-rural settings, we don't put up traffic signals in rural areas like I've seen in the US.

[jakeroot]

we tend to avoid this situation by building roundabouts, non-signalized intersections or simple grade-separated interchanges instead, especially in rural or semi-rural settings, we don't put up traffic signals in rural areas like I've seen in the US.

Thankfully, it seems like some states are ending this practice. I can't think of any new rural signals in Washington state in recent memory. Almost all [rural] junction reconfigurations have been roundabouts, so far as I am aware.

[roadman65]

Sweden requires ... Also, on roads with a speed limit of higher than 70 km/h (45 mph) traffic signals are prohibited.

Now this I find intriguing. In the US, there are plenty of roads with speed limits higher than that where a traffic signal may be encountered. Signals on roads posted at 65 definitely exist and I would be surprised if someone found one on a road posted at 70 (although I can't think of an example offhand).

Does this policy mean that a high speed road will be underposted at 70 km/hr if a traffic signal is present? Or does it mean that the relevant authorities will go to great lengths to avoid this situation?

I don't think Florida will put signals on roads set at 65mph (Florida's maximum for non-highways). When they need a signal, they post a 60mph (or sometimes 55mph in some counties) sign about 1000ft before the intersection, and then raise it back up 1000ft after it.

Actually in FDOT District 1 they do have 65 mph through signalized intersections.  Look at US 27 at State Road 29 near Palmdale where there is a signal there and US 27 is 65 mph.

Also in Polk County before the US 27 speed limit was lowered to 60 mph after the sprawl kicked in there, it was 65 through SR 542 near Cypress Gardens and through CR 547 in Haines City.

[steviep24]

Sweden requires ... Also, on roads with a speed limit of higher than 70 km/h (45 mph) traffic signals are prohibited.

Now this I find intriguing. In the US, there are plenty of roads with speed limits higher than that where a traffic signal may be encountered. Signals on roads posted at 65 definitely exist and I would be surprised if someone found one on a road posted at 70 (although I can't think of an example offhand).

Does this policy mean that a high speed road will be underposted at 70 km/hr if a traffic signal is present? Or does it mean that the relevant authorities will go to great lengths to avoid this situation?

I don't think Florida will put signals on roads set at 65mph (Florida's maximum for non-highways). When they need a signal, they post a 60mph (or sometimes 55mph in some counties) sign about 1000ft before the intersection, and then raise it back up 1000ft after it.

Actually in FDOT District 1 they do have 65 mph through signalized intersections.  Look at US 27 at State Road 29 near Palmdale where there is a signal there and US 27 is 65 mph.

Also in Polk County before the US 27 speed limit was lowered to 60 mph after the sprawl kicked in there, it was 65 through SR 542 near Cypress Gardens and through CR 547 in Haines City.

In NY the western terminus of NY 531 has 65 MPH facing a traffic signal.

https://www.google.com/maps/@43.1864935,-77.8538159,3a,75y,292h,90t/data=!3m6!1e1!3m4!1sPCi1dxQ7zcI88r3MTtezzw!2e0!7i13312!8i6656?hl=en

[cpzilliacus]

Does this policy mean that a high speed road will be underposted at 70 km/hr if a traffic signal is present? Or does it mean that the relevant authorities will go to great lengths to avoid this situation?

It means that speed will be lowered to 70 km/h in time before encountering the signal, but also that we tend to avoid this situation by building roundabouts, non-signalized intersections or simple grade-separated interchanges instead, especially in rural or semi-rural settings, we don't put up traffic signals in rural areas like I've seen in the US.

Agreed.

Rural, "in the middle of nowhere," signalized intersections in the Nordic nations are rare, and as Riiga says, there has been some effort devoted to getting rid of those when possible. 

A longer example of this is National Highway 2 in Finland, which runs on a northwest/southeast axis between the municipality of Pori (Björneborg in Swedish) and an interchange with National Highway 1/Route E18 in the western exurbs of Helsinki.

It once had a lot of signalized at-grade intersections, but most have been replaced by simple grade separations like this (traffic volumes do not warrant a full Super-2 type of road).

[vdeane]

Does this policy mean that a high speed road will be underposted at 70 km/hr if a traffic signal is present? Or does it mean that the relevant authorities will go to great lengths to avoid this situation?

It means that speed will be lowered to 70 km/h in time before encountering the signal, but also that we tend to avoid this situation by building roundabouts, non-signalized intersections or simple grade-separated interchanges instead, especially in rural or semi-rural settings, we don't put up traffic signals in rural areas like I've seen in the US.

I wonder if Quebec does something similar.  That segment of A-30 near Becancour does the same thing.

Though around here, signalized rural intersections are relatively common and rural roundabouts are rare.