Guidelines for Timing Yellow and All-Red Intervals

I appreciate your providing this link. It provides us with valuable information.

I would be interested in your thoughts on the 85th percentile.

What this says to me is that people are (generally) speeding to begin with (the research suggests adding 7 MPH to the posted speed limit to get the true approach speed).

What this also says to me is that if a municipality were to change posted speed limits to the 85th percentile speed, then - within a few months - the new 85th percentile speed would be 7 MPH greater that the previous one - 14 MPH greater than the original posted speed limit.

Thinking about roads in residential areas (or near them), I think that this would result in increased accidents and endanger or kids. I say this because I am aware of my own tendency to continue to travel at my previous "average" speed because reducing my speed is not very easy to do. My mind has gotten used to the sensations produced my an extended time at a certain speed and balks at reducing it very much - although I try to do so.

What are your thoughts?

On the whole, the average driver is very poor of making that type of decision. They tend to travel at speeds that "feel right" to them with little to no understanding of other factors such as sight distances and other factors considered in the original engineering for the road.

Are some limits too low, of course. Sometimes limits are set by local ordinance and that is also factored into the design of the road. If the local ordinance states the maximum speed limit that is allowed is say 35, then the road is engineered for that speed. For the most part, speed limits were set before 1965 and really haven't changed much since then. Only a few of our limited access roads have been designed for sustained speeds above 60 even though most Interstates have posted limits of 65 and drivers tend to push their cars to 70 and 75.

Design of roads also hasn't kept pace with the improvements in vehicles either. In 1965 the only safety features in all cars were collapsible steering wheels and seat belts - not shoulder but lap belts. Tires have changed, brake systems have changed from 4 wheel drum to at least front disc with many having 4 wheel discs. The design criteria for roads is changing, but it takes 50 years for a change to be implemented just because that's how long a bridge or road is designed to last.

imho there has to be a judgement factor. It's zany to think that everything is black and white. People cannot even alternate merge in Phila. Yet, the Lincoln Tunnel inbound has 12 lanes merging into 3, with no stopping, and total alternation. In LA, it's so laid back, people have all day to come up to speed, and they don't expect to be boxed-out.

There is no one size fits all. However, it doesn't take a genius to know whether or not he's run a red light. He did, or he didn't. Light turns red when you're over the stop line, you ran it.

running a red light is encouraged in places where, in their infinite wisdom, cycle time is set at an absured 5 min. (as some lights in Raleigh, NC) or another time deemed too long by most drivers. The light starts to change and it is pedal to the medal to avoid the wait.

There is NEVER one reason for something like this. Studies need more input.

I have little faith that all those guide lines will assist a city in adjusting the yellows. There are too many variables.
KISS: Keep it simple stupid.
30 mph = 3 seconds
40 mph = 4 seconds
Then let the city engineer or a staff member personally observe traffic at busy and dangerous intersections. Then empower them to make changes as required.

This is from an old NHTSA study but not much has changed. (Ok, fuel is now more expensive and cars are safer).

The quick synopsis:
Speed limits are set too low in general, a practice which actually causes UNSAFE conditions.

Raising the speed limit has no significant effect on the prevailing travel speed, either on those roads or on secondary roads.

Now I should probably add that this study was performed back when speed limits in general were truly ridiculous. Many states have raised the speed limits on their interstates to a somewhat more reasonable level.

You almost never see anything above 75 though and probably never will because it just sounds too fast. 80 mph isn't much faster than 75 mph and there are many sections of highway where an 80-85 mph speed limit would be completely safe and reasonable but hearing someone say that they were driving 80+ mph evokes an emotional response. The first thought that goes through most people's minds is probably "idiot".

- Phil

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From:
http://www.ibiblio.org/rdu/sl-irre0.html

Preliminary Results
Driver compliance with speed limits is poor. On average, 7 out of 10 motorists exceeded the posted speed in urban areas. Compliance ranged from 3 to 99 percent. Compliance tended to be worse on low-speed roads, better on roads with prima facie limits, or where the speed limit was based on an engineering study. Better does not mean good compliance; less than 10 percent on [sic] the sites had more than 50-percent obedience with the posted speed.

On many streets and highways the speed limit is set 8 to 12 mph below the prevailing 85th-percentile speed. The extreme case was a prevailing speed of 51 mph in a 30-mph zone. Truck speeds were consistently 3 mph slower than car speeds in urban areas. The factors that had the most influence on speeds were number of access points and commercial development.

The accident involvement rates on streets and highways in urban areas was highest for the slowest 5 percent of traffic, lowest for traffic in the 30-to-95-percentile range and increased for the fastest 5 percent of traffic. The relative involvement rate is a measure of the chance of being involved in an accident, and is a ratio of the percent of accidents in a given speed range to the percent of travel in the same speed range.

For each accident that occurred at a site, the speed of each vehicle involved in the accident was assigned to the appropriate percentile speed category for that site. All such data from each site were then combined and the relative risk computed. The risk curve for roads in built-up areas is consistent with the work of Solomon, Cirillo, and West, and Dunn which showed that the risk of involvement in accidents is minimum near the average speed of traffic and increases dramatically for vehicles traveling much slower or faster than average. The rate at which drivers experience overtakings follows a similar U-shaped relationship and provides a theoretical explanation for the shape of the speed-risk curve.

Many current speed limits coincide with 30-percentile speed, which is near the lower bound of safe travel speed. Speed limits should be set in the 70-to-90-percentile range or roughly 5 to 10 mph above the average speed to correctly reflect maximum safe speed. Speed limits are set in multiples of 5 mph; the 70-to-90-percentile range will almost always include a 5-mph multiple. Allowing a 5-mph tolerance, enforcement would then be targeted at drivers who are clearly at risk. If speed limits were raised to more realistic levels, would drivers automatically drive 5 to 10 mph over the new speed limit as is commonly believed? The answer is no. Raising the speed by various amounts up to 15 mph has little or no effect on speeds over a broad range of road types and speed levels.

Conversely, lowering the speed limit will not slow down traffic. Although speed increases of 3 mph and decreases of 3 mph were observed at individual sites, the expected change in speed is less than 1 mph, which is normal variation. In addition, there is no evidence in our studies that raising the speed limit to 65 on rural interstate freeways led to an increase in speeds off the freeway.

I regularly approach an intersection that has a countdown timer and there is also a speed/red light camera.

I have yet to discover when the light will turn yellow. Sometimes the light turns yellow when the countdown reaches 0 and at other times the yellow appears after an apparently random interval based on traffic in multiple directions.

Unless you assume that a countdown of 0 means yellow and are prepared to stop you are always faced with the decision as to when the yellow will really come on. In those circumstances you are faced with the possibility that the person behind you has a different mindset.

It's pretty hard to judge timing when the timing changes different times of the day. I know I'll get arguments on my take on it but other countries are using the system and it must be working. Instead of a yellow light replace it with a timer that starts a countdown the minute the green comes on. We have one light in my area on a road with a 55 MPH speed limit and it seems there is no set timing on it, from one day to the next the timing changes. This would be a great place for such a light.

All the data gathered is correct, but that is only if it applied to a properly set up intersection.. The municipalities re-engineered the intersections by moving the stop line inwards towards the center of the cross street and made the crosswalks narrower. This gives the driver less time to react to the light and be within the intersection when the light turns yellow.. no matter what your speed is when the light goes into the yellow ( R x T = D ) phase you are still outside the intersection and trigger the sensor pad when it cycles to the red phase.. You get the ticket.. If the stop line was moved back you would be in the intersection when the light turned yellow and would have enough time to clear the crosswalk within the 3 seconds of the yellow phase and thru the red phase.. No money for the local gov't...

Not true in all states. That's one problem right there.

For some of the other poster's comments, don't forget that an 'all directions red' time is always an option for planners should they want to choose safety over traffic flow in certain cases.