Why 85th percentile

Problem solved. It was designed so vehicles had to drive in the cycle lane as they were concerned about head on collisions between vehicles at 40mph. It was suggested that if they dropped the speed limit then lane widths could be reduced to allow a mandatory bit of paint. It cuts both ways: as a tool it can be used to either justify adding traffic calming measures, or just increasing the speed limit. So if the planners are ignoring cyclists they are also ignoring the most obvious speed differential.

In Toronto, where I live, the 85th percentile been used both ways. The transportation department has made it quite difficult for communities to reduce the speed of vehicles on their street.

Resources for installing traffic calming measures are limited and a community is required to jump through hoops before anything is installed. The 85th percentile is used to justify that the status quo is fine. Only recently has the downtown community council representing municipal politicians downtown been able to reduce the speed limit on all residential streets. By making a blanket reduction, the communities can then request traffic calming measures from the transportation department by using the 85th percentile.

Mysteriously, the attitude of TPTB when it comes to cycling-specific signage is exactly the opposite—with London city council at the vanguard of strongly discouraging signs which are legible to even stationary cyclists equipped with binoculars…. Some traffic calming methods have the presumably unintended side-effect of slowing cyclists down too.

I think this is an important piece. Riding in France this year in a wide range of environments and landscapes the road-design and not signage or driver-choice was the greatest contributory factor limiting speed, choice and it seemed to me stress upon all road-users making my time cycling there unrecognisable from cycling in the British Isles.

Perhaps proper punishment of offeders would be a cheaper alternative, tjough some mix of design and serious enforcement might be best. The shame of it is that most Local Highway Authorities have at the very start of their Design Guides a heirarchy of roads. But it actually just makes them look like Luddites who have too much time on their hands and could do with a few rounds of thinning-out.

I still have the occasional chuckle at the time I was on my bike following some car in a 20 mph street in Strawberry Hill where those lopped off pyramid-stylee road bumps were used to effect speed reduction.

The driver bottomed out the car on the tarmac over every single one: they never did get the timing of the accleration-braking right. Many have zebra crossings on top. Would other interventions do better? Not sure. Still, any intervention should be able to handle the diffcult cases, wherever they occur.

However, it also has these wide table-top speed bumps. The speed bumps do not extend across the entire street, The green painted gaps at either side are for cyclists to ride through without having to slow down, and the little traffic islands are to prevent parking at those points so that cyclist have the necessary room. That never fails to surprise them : —.

I think that some drivers possibly need some training as to how to drive in 20 mph zones. Changes to the driving landscape such as 20 mph zones could be a good reason for introducing re-testing at intervals in a drivers career to ensure they are up to date with such changes.

Including things that are covered in the Think! The exciting thing about dropping the limit is that the 85th percentile can then be posed as a problem. For example, the Royal College Street upgrade proposal noted that the 85th percentile speed was around 30mph I forget the exact figure.

Given the speed limit was now 20mph, this was a problem. That is the argument used by our downtown Toronto politicians who recently dropped the residential street limit to 30kph. Perks, one of the politicians said that now the 85th percentile will now show that traffic calming is required. A catch 22 that allows them to do nothing. So looks like the solution was political to force it open.

The volume plays a larger factor though. Keeping the speed low is a worthy goal though. This also makes me think about motorway speed limits. The traffic is mostly going that speed anyway and crash risks are lowest when traffic moves at the same speed, regardless of exactly what that speed is.

The Netherlands has a very extensive path network throughout the country so that people only rarely need to bike on roads and even then only on roads with low traffic and low speeds. On my last trip to visit relatives in the Netherlands, my wife and I bought the thick guidebook of the path network and used it extensively in South and North Holland.

And the Raleigh Connoisseur reports that a major increase in bus service goes into effect this week in Wake County, following a November vote to increase the local sales tax half a cent. Facebook Twitter Google Plus Email print.

The "85th Percentile Rule" relies on outdated assumptions and needs to be fixed. More than , people lost their lives in speed-related crashes from to , accounting for 31 percent of all traffic deaths in America over that period.

In a draft report released earlier this week, the National Transportation Safety Board says excessive speed is a deadly problem in our nation's transportation system -- one that federal and state officials aren't doing enough to address. This research is important because it informs the way we set speed limits in the state of California and in many other states across the country.

The topic of speed limits is of current interest to the California legislature. Speed is a key factor in roadway safety and speed limits have a great influence on the roadway speed. What are the criteria for installing speed humps or other modifications even when the 85th percentile speed seems within reason?

It is logical that strict enforcement will tighten the driving speed around the speed limit being enforced, likely lowering both the average and 85th percentile speed.

Thanks for the reply. Please send me any research you find. Mike, Thanks for the reply. Interestingly enough the vast majority of residents addressing the Commission complain about high traffic speeds on their street.

Best Regards, -Henry. Hi Henry — The 85th percentile speed is a statistical construct. In our town there are a number of streets that now have speed humps. Other streets with humps have the same classification on CRS. I suspect that the other streets that got the speed humps installed managed to get enough people to attend the Commission meetings to make their voice heard and the City responded.

I can empathize with the engineering approach but I think this situation is more physiological in nature than many traffic engineers thinks it is. I think this is a trend — e. Berkeley is an example. People want their neighborhood to be more livable. Care must be taken when collecting data on a two lane two-way roadway. Typically the direction given to those collecting the data to only collect the speeds of free flowing vehicles.

This means that in a group only the speed of the lead vehicle is then recorded. Ideally, if there are a lot of following vehicles, the speed data should then be collected during lighter traffic flows. I got an email indicating additional activity on this thread. Can you provide a link? Since my last posting here an additional speed survey was done on my street. The 85th speed went up during a period of no enforcement.

I have my doubts that the 85th percentile rule holds for a residential street with cross streets, pedestrians from two nearby schools and cyclists. Thanks, -Henry. Thanks for the comment. This is very clear — thank you. Some would say 65 to account for the probable increase in the actual 85th percentile speed of 1 mph after a posted increase of 5 mph. Note that limits at the 85th percentile speed tend to be the safest with the fewest crashes.

The most extensive research ever done on what happens with increased and decreased posted limits was done for the FHWA in by Martin Parker and Associates. You can raise a too-limit by up to 15 mph, or lower a good one by up to 20 mph — and the actual 85th percentile speed will change by no more than 3 mph, but usually by 0 to 2 mph with an average change of 1.