85th percentile speed and speed reduction

[sidman69]

Hi guys,

I was doing a speed data assessment along a highway whose speed is 100km/h. I calculated the 85 percentile along three locations I setup to collect the speed data to be about 104-105km/h with the mean being 99-100km/h. We were looking at lowering the speed to 80km/h due to a lot of pedestrians crossing the highway and we have had pedestrians killed at least 5 times due to them crossing the highway. The highway runs through an indian reserve. There are also a high number of accesses on to the highway, something like one access per 300m, which is quite high.

Based on the data I collected it seems like theirs moderate compliance with the speed limit and lowering the speed limit to 80km/h wouldn't ensure that their would be compliance and as it stands the existing speed limit is satisfactory. Am I correct in making this assumption?

 

[ACtrafficengr]

There is an FHWA paper from some years ago that found the speed limit has little impact on actual travel speeds. It can have an effect on safety, but I'm not sure if they make a distinction whether the speed limit is lowered to the 85th % speed, or lower than the 85th % speed.

http://www.fhwa.dot.gov/tfhrc/safety/pubs/speed/speed.htm

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[ABKaminski]

The common rule I follow is +/- 5 miles per hour (~ 8 kph!) from the 85th percentile speed OR if you are looking for a posting lower than this you will want to install some physical means (traffic calming) to enforce the lower speed limit. The general idea is that most motorists (85 percent of them!!) will travel at a safe and prudent speed and the speed limit should reflect that speed (hence posting a speed at +/- 5 miles per hour from the 85th percentile speed)
I hope this helps--

 

[sidman69]

thanks for the replies. Just some more info i have. Most of the collisions involving pedestrians have occured at night with pedestrians crossing at unlit spots and most accidents are not due to people speeding. There already are numerous watch for pedestrian signs so drivers are aware of pedestrians crossing the highway. Although it seems like 4-5 pedestrians have been killed at night along a horizontal curve. This would make sense since a person driving along the curve at night probably can't see more than 20 - 30 m ahead of them based on where there headlights are projected.

 

[ACtrafficengr]

Sounds like highway lighting is in order.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[sidman69]

i considered highway lighting, but the study section is 14-15km right now and the department is looking for a short term solution until pedestrian underpasses are constructed