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Speed Limits - 85 percentile

Posted: Sun Oct 14, 2012 8:17 pm
by mikesbytes
Interesting piece of logic, but how does it translate to determining speed limits in the real world?

The 85th percentile

Re: Speed Limits - 85 percentile

Posted: Sun Oct 14, 2012 11:17 pm
by high_tea
mikesbytes wrote:Interesting piece of logic, but how does it translate to determining speed limits in the real world?

The 85th percentile


The only criterion discussed seems to be crash frequency. Such being the case I'd say, Badly.

Re: Speed Limits - 85 percentile

Posted: Mon Oct 15, 2012 4:18 am
by Strawburger
The road authority sets the speed, the road designer (engineering team) designs the road to confirm to the set speed limit. The constructor build the road and installs the road furniture (sign barriers etc). The road authority then operates the road, or hands it over to council or private investor. It is in this period where the posted speed may fluctuate from designed speed (usually down if they modify the road or add in objects surrounding the road).

There are volumes and volumes of road design guides that determine speed limits from geometric shape of the road, stormwater drainage, signage, soil types, bridges, costructability, safety and (a large amount of time) cost. I won't bore you with all that! If there is enough interest I can give a rundown of it.

That is the science behind it, loads of maths involved. It's not as easy as looking at a road and saying this could be xxx km/hr or arguing that my car can do xxx km/hr more or im a great driver so I should be able to do more

Re: Speed Limits - 85 percentile

Posted: Mon Oct 15, 2012 8:07 am
by find_bruce
I stopped reading the article fairly quickly at the number of assertions that higher speed correlates to higher skill.

As for Strawburgers post, the only issue I have with it is that there is a gap between theory and practice. It could be poor design, but of course it could also be the road was not built as designed or that drivers are idiots who fail to grasp simple concepts

Re: Speed Limits - 85 percentile

Posted: Tue Oct 16, 2012 3:25 pm
by tubby74
I've had some rapid lessons in all this stuff as I am fighting over speed humps installed outside my house just now. The justification by RMS for this was the 85th percentile was 46 km/h in a 50 zone. somehow that meant we needed speed humps and a 40 zone to stop the people who were not speeding to start with.
There's almost no leeway in the guidelines for common sense - to get a pedestrian crossing you have to multiply # of cars in a 3 hour period by # of pedestrians. Asking if the road is so dangerous we need speed humps can we convert one of those into a pedestrian crossing so the kids can use it on their way to school - no it's not dangerous enough for that.

The guidelines exist based on a lot of research but the lack of ability to plan well around them is incrdeibly frustrating.

Re: Speed Limits - 85 percentile

Posted: Tue Oct 16, 2012 3:47 pm
by Sweeper59
Tubby, the RMS and local Councils get 100's of requests for pedestrian crossings every year. The requests come from all sorts of sources for a variety of reasons. The authorities have to weigh up the benefits of a pedestrian crossing against the cost of the infrastructure and the impacts on traffic flows. As you mentioned, there are guidelines based on vehicle and pedestrian volumes, to determine the requirement for a pedestrian crossing. Without these guidelines, ped crossings locations would simply be determined by the loudest resident group, the biggest retailer or the local councillor with best RMS contacts.

Re: Speed Limits - 85 percentile

Posted: Tue Oct 16, 2012 4:20 pm
by tubby74
i don't disgaree with guidelines, but it's the complete lack of discretion around them.

Today I've talked to a regional manager at RMS, council development manager, and the traffic engineers from council came out to visit my street. Whilst I was talking to them someone from further up the road came out to talk about his problems with the speed humps. Everyone I talked to agreed we'd prefer if we could remove the speed humps, yet guidelines say no. The guidelines are also based only on easily measured numbers, basically speed and traffic counts. the fact that cars swerve dangerously through the speed humps, or take a corner dangerously to avoid them is not something considered, and since it's not in the guidelines there's no way to apply discretion to fix it.

Re: Speed Limits - 85 percentile

Posted: Tue Oct 16, 2012 10:44 pm
by lturner
mikesbytes wrote:Interesting piece of logic, but how does it translate to determining speed limits in the real world?

The 85th percentile


If you think about it carefully the 85th percentile theory actual suggests that speed limits should be set at the level at which it actually has no effect whatsoever in influencing the speed of the traffic. In fact sometimes it is assumed that speed limits don't ever have an effect on driving speeds, ie:

"The majority of motorists drive at a speed they consider reasonable, and safe for road, traffic, and environmental conditions. Posted limits which are set higher or lower than dictated by roadway and traffic conditions are ignored by the majority of motorists."

I think this is complete rubbish. When residential speed limits were generally lowered from 60km/h to 50km/h, did this have no effect on the speeds people were driving?

Re: Speed Limits - 85 percentile

Posted: Tue Oct 16, 2012 11:08 pm
by Howzat
lturner wrote:I think this is complete rubbish.


Yep, me too. A lot of argument-by-assertion in that article; sounds like someone with too much time on their hands got a speeding ticket. In the part you cite, the argument is also tautological, because "a speed they consider reasonable" is indicated in large part by posted speed-limit signs, especially in built-up areas. They're not just rules - they're information as well.

Re: Speed Limits - 85 percentile

Posted: Wed Oct 17, 2012 9:53 am
by mikesbytes
Regardless whether its good logic or rubbish, I don't understand how the logic translates into calculating the speed limit.

Re: Speed Limits - 85 percentile

Posted: Wed Oct 17, 2012 12:14 pm
by Comedian
I think he idea of this is that broadly people pick a safe speed and will go with it. The speed lit should be set accordingly and only book people that are traveling above the norm.

I actually think this wasn't such a bad idea in the olden days when the roads were sparsely populated. It doesn't work now.

Re: Speed Limits - 85 percentile

Posted: Wed Oct 17, 2012 12:53 pm
by simonn
Comedian wrote:I actually think this wasn't such a bad idea in the olden days when the roads were sparsely populated. It doesn't work now.


You mean the old days wen the roads were more sparsely populated, but more people (in actual terms) died on them?

Re: Speed Limits - 85 percentile

Posted: Wed Oct 17, 2012 1:51 pm
by Comedian
simonn wrote:
Comedian wrote:I actually think this wasn't such a bad idea in the olden days when the roads were sparsely populated. It doesn't work now.


You mean the old days wen the roads were more sparsely populated, but more people (in actual terms) died on them?

yep, and the ones without seatbelts and were a 25kph crash was barely survivable. :)

Re: Speed Limits - 85 percentile

Posted: Wed Oct 17, 2012 2:47 pm
by Comedian
Oxford wrote:
simonn wrote:
Comedian wrote:I actually think this wasn't such a bad idea in the olden days when the roads were sparsely populated. It doesn't work now.


You mean the old days wen the roads were more sparsely populated, but more people (in actual terms) died on them?
the drop is largely attributed to more safety features available now (eg ABS, SRS etc), safer environments (eg vehicle cabin construction and materials), better quality equipment (eg tyres, shocks etc) and compulsory seat belt laws. but lets not let facts get in the way of a good argument hey.

yep those were the days. They were also when you drove home from the pub because you were too drunk to sit at the bar... Of course I'm too young to remember this. :mrgreen:

Re: Speed Limits - 85 percentile

Posted: Wed Oct 17, 2012 2:52 pm
by zero
mikesbytes wrote:Regardless whether its good logic or rubbish, I don't understand how the logic translates into calculating the speed limit.


The web pages logic test fails badly. ie their belief that motorists ignore speed limits is erroneous. The 85th percentile will however be above or below the speed limit depending how safe motorists feel the road is on average. The magnitude of it being above or below will not be massive unless there is absolutely no enforcement. Its a stronger test of when a speed limit is too high, than when it is too low.

Also their accident speed logic is terrible. Most motorway accidents don't happen because a motorist driver was driving at 80 km/hr, they happen because the traffic slowed due to a congestion effect or weather effect. ie many of the drivers in their "sub speed" graph, are not drivers who "intended" to travel at that speed, and who probably spent the majority of their trip at or near speed limit. The motorway has to be more than 50% full - ie 6 second average headway between cars before an 80km/hr car is even an impediment that might cause congestion.

Re: Speed Limits - 85 percentile

Posted: Wed Oct 17, 2012 6:22 pm
by Strawburger
A lot of calculations are based around safety of the road and driver behaviors. Let's not go back to sub standard roads where deaths were frequent and technology was not as advanced. Standards are updated from time to time to allow for technological advances by the way.

I'll give you all one example of one calculation used and how a safe road can become an unsafe one, and has no bearing on how good a driver is.

Driver heads over a crest curve (going uphill and flattening out) on a 2 lane motorway. An animal wanders onto the road just beyond the crest. In order to see the animal (which for arguments sake is 200mm high) the sight distance needed to spot the animal, to react to hit the brakes and to pull up just short of the animal is 210m. Now this distance significantly increases as the speed increases.

In the tight windy areas of northern NSW, the road designers are sometimes forced to use this minimum values in order to fit the geometric limitations into their design.

Ok, we can argue that the animal may flee before reaching it or vehicles can swerve to avoid it. Sometimes there are other objects on the road such as large trees spanning the road, or perhaps a motorcyclist laying on the ground after an accident. How is this accident avoidable when the car is doing "safe speeding", there is debris on the road (oil even?) and the driver is distracted leading up to the area in question?

This is just one example of one calculation. There are way too many other examples that attempt to eliminate the risk of crashes on the road and involve all the full 100 percentile.

Re: Speed Limits - 85 percentile

Posted: Wed Oct 17, 2012 9:43 pm
by tubby74
lturner wrote:
I think this is complete rubbish. When residential speed limits were generally lowered from 60km/h to 50km/h, did this have no effect on the speeds people were driving?


What was the 85th percentile on those shot of streets before? I'd be surprised if it was that high, most 50 streets now weren't ever major thoroughfares.
I think a good example of his theory is Anzac bridge in Sydney. 6-8 lanes used to be a 70 zone. Now it is 60 yet try sitting on that speed. You'll have drivers tailgating and swerving round you to get by. The majority decide 70 or just over is still the appropriate speed

Re: Speed Limits - 85 percentile

Posted: Wed Oct 17, 2012 10:37 pm
by zero
tubby74 wrote:
lturner wrote:
I think this is complete rubbish. When residential speed limits were generally lowered from 60km/h to 50km/h, did this have no effect on the speeds people were driving?


What was the 85th percentile on those shot of streets before? I'd be surprised if it was that high, most 50 streets now weren't ever major thoroughfares.
I think a good example of his theory is Anzac bridge in Sydney. 6-8 lanes used to be a 70 zone. Now it is 60 yet try sitting on that speed. You'll have drivers tailgating and swerving round you to get by. The majority decide 70 or just over is still the appropriate speed


People drive at 10 over, because its hard to lose your licence doing so. IMO the change to 50 eventually brought most of those people down to 60, and ensured that 70 in a suburban street was well into the serious punishment range (as it should be). I've no doubt the 85th percentile for some suburban streets is above 50, but I also have little doubt that the speed was lowered by the speed limits over time.

85th percentile of Anzac bridge is probably 40km/hr ish I would think. No chance of it ever getting over 70.

Re: Speed Limits - 85 percentile

Posted: Wed Oct 17, 2012 11:02 pm
by Howzat
tubby74 wrote:85th percentile of Anzac bridge is probably 40km/hr ish I would think. No chance of it ever getting over 70.


Average vehicle speed on the main city-bound roads in the morning in Sydney is about 30 km/h. :|

I'm guessing 85th percentile is about 31 km/h. :roll:

Re: Speed Limits - 85 percentile

Posted: Thu Oct 18, 2012 4:50 am
by Strawburger
tubby74 wrote:
lturner wrote:
I think a good example of his theory is Anzac bridge in Sydney. 6-8 lanes used to be a 70 zone. Now it is 60 yet try sitting on that speed. You'll have drivers tailgating and swerving round you to get by. The majority decide 70 or just over is still the appropriate speed


Incidentally, the speed lowered due to altering the layout to attempt to fix congestion, thus lowering the safety standards.

Re: Speed Limits - 85 percentile

Posted: Thu Oct 18, 2012 8:58 am
by mikesbytes
Straight line reaction and braking distance is a documented metric. Using this example, how is the required stopping distance on a piece of road determined?

Re: Speed Limits - 85 percentile

Posted: Thu Oct 18, 2012 9:52 am
by find_bruce
On the city bound roads in Sydney I ride along each morning, while the maximum achieved speed may be in region of 30-35 km/h, it will take around 30 minutes to drive the 6.1 km from the corner of Victoria Rd & Lyons Rd Drummoyne to the corner of King St & Sussex St in the city, average speed is more like 15 km/h

Should I note that if the advisory signs on Pyrmont Bridge were set to 85% of cylists speeds, the number would be a lot higher than 10.

Re: Speed Limits - 85 percentile

Posted: Thu Oct 18, 2012 7:26 pm
by Strawburger
mikesbytes wrote:Straight line reaction and braking distance is a documented metric. Using this example, how is the required stopping distance on a piece of road determined?


Going from memory here, I will look it up when i am back at work.

Reaction time is set depending on type of road and surrounding either 1.5 or 2.5 seconds), then the stopping distance looks at type of road surface, vehicle type and weight, rate of deceleration then there is a slope (grade) of road factor. Once you have that distance then you need to calculate that distance linearly to the road geometry (which is generally curved) from the drivers eye height (varies between states of aust but between 1.1m and 1.2m from the ground) and eye offset from lane (usually 1.5m) to the linear object target which in this example is 0.2m (but can vary depending on what is calculated : object height, lane line height 0.0m, tail light which is 0.6m or vehicle which is eye height and all these values change the calculated distances).

Does that cover what you are asking?

Re: Speed Limits - 85 percentile

Posted: Thu Oct 18, 2012 9:26 pm
by tubby74
For the example of Anzac bridge I was thinking weekend and off peak times when traffic is flowing, its always well over the speed limit. Obviously at peak times neither the speed limit nor a drivers own choice is the limiting factor in speed.

Re: Speed Limits - 85 percentile

Posted: Tue Oct 23, 2012 11:35 am
by mikesbytes
Strawburger wrote:
mikesbytes wrote:Straight line reaction and braking distance is a documented metric. Using this example, how is the required stopping distance on a piece of road determined?


Going from memory here, I will look it up when i am back at work.

Reaction time is set depending on type of road and surrounding either 1.5 or 2.5 seconds), then the stopping distance looks at type of road surface, vehicle type and weight, rate of deceleration then there is a slope (grade) of road factor. Once you have that distance then you need to calculate that distance linearly to the road geometry (which is generally curved) from the drivers eye height (varies between states of aust but between 1.1m and 1.2m from the ground) and eye offset from lane (usually 1.5m) to the linear object target which in this example is 0.2m (but can vary depending on what is calculated : object height, lane line height 0.0m, tail light which is 0.6m or vehicle which is eye height and all these values change the calculated distances).

Does that cover what you are asking?


Yes, except I suspect that the type of car wouldn't be in the calculation, as they would need to use the poorest performing vehicle for the calcuation