Speed Limits - 85 percentile
- mikesbytes
- Super Mod
- Posts: 22183
- Joined: Thu Dec 01, 2005 11:42 pm
- Location: Tempe, Sydney
- Contact:
Speed Limits - 85 percentile
Postby mikesbytes » Sun Oct 14, 2012 8:17 pm
The 85th percentile
-
- Posts: 1494
- Joined: Thu Apr 29, 2010 8:10 pm
- Contact:
Re: Speed Limits - 85 percentile
Postby high_tea » Sun Oct 14, 2012 11:17 pm
The only criterion discussed seems to be crash frequency. Such being the case I'd say, Badly.mikesbytes wrote:Interesting piece of logic, but how does it translate to determining speed limits in the real world?
The 85th percentile
- Strawburger
- Posts: 1729
- Joined: Tue Aug 26, 2008 12:19 pm
- Location: Dulwich Hill, Sydney
Re: Speed Limits - 85 percentile
Postby Strawburger » Mon Oct 15, 2012 4:18 am
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
- find_bruce
- Moderator
- Posts: 10615
- Joined: Mon May 09, 2011 8:42 pm
- Location: Sydney
Re: Speed Limits - 85 percentile
Postby find_bruce » Mon Oct 15, 2012 8:07 am
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
-
- Posts: 375
- Joined: Mon Aug 22, 2011 7:02 pm
Re: Speed Limits - 85 percentile
Postby tubby74 » Tue Oct 16, 2012 3:25 pm
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.
- Sweeper59
- Posts: 826
- Joined: Wed Oct 28, 2009 1:37 pm
- Location: Port Macquarie NSW
Re: Speed Limits - 85 percentile
Postby Sweeper59 » Tue Oct 16, 2012 3:47 pm
Nobody looks back on their life....and remembers the nights they got plenty of sleep !!
-
- Posts: 375
- Joined: Mon Aug 22, 2011 7:02 pm
Re: Speed Limits - 85 percentile
Postby tubby74 » Tue Oct 16, 2012 4:20 pm
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.
-
- Posts: 198
- Joined: Tue Jun 21, 2011 2:11 am
Re: Speed Limits - 85 percentile
Postby lturner » Tue Oct 16, 2012 10:44 pm
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:mikesbytes wrote:Interesting piece of logic, but how does it translate to determining speed limits in the real world?
The 85th percentile
"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?
- Howzat
- Posts: 850
- Joined: Wed Aug 15, 2012 7:08 pm
Re: Speed Limits - 85 percentile
Postby Howzat » Tue Oct 16, 2012 11:08 pm
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.lturner wrote:I think this is complete rubbish.
- mikesbytes
- Super Mod
- Posts: 22183
- Joined: Thu Dec 01, 2005 11:42 pm
- Location: Tempe, Sydney
- Contact:
Re: Speed Limits - 85 percentile
Postby mikesbytes » Wed Oct 17, 2012 9:53 am
- Comedian
- Posts: 9166
- Joined: Mon Aug 09, 2010 7:35 pm
- Location: Brisbane
Re: Speed Limits - 85 percentile
Postby Comedian » Wed Oct 17, 2012 12:14 pm
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.
- simonn
- Posts: 3763
- Joined: Thu Jan 31, 2008 11:46 am
- Location: Sydney
Re: Speed Limits - 85 percentile
Postby simonn » Wed Oct 17, 2012 12:53 pm
You mean the old days wen the roads were more sparsely populated, but more people (in actual terms) died on them?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.
- Comedian
- Posts: 9166
- Joined: Mon Aug 09, 2010 7:35 pm
- Location: Brisbane
Re: Speed Limits - 85 percentile
Postby Comedian » Wed Oct 17, 2012 1:51 pm
yep, and the ones without seatbelts and were a 25kph crash was barely survivable.simonn wrote:You mean the old days wen the roads were more sparsely populated, but more people (in actual terms) died on them?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.
- Comedian
- Posts: 9166
- Joined: Mon Aug 09, 2010 7:35 pm
- Location: Brisbane
Re: Speed Limits - 85 percentile
Postby Comedian » Wed Oct 17, 2012 2:47 pm
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.Oxford wrote: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.simonn wrote:You mean the old days wen the roads were more sparsely populated, but more people (in actual terms) died on them?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.
-
- Posts: 3056
- Joined: Tue Jan 05, 2010 2:54 pm
Re: Speed Limits - 85 percentile
Postby zero » Wed Oct 17, 2012 2:52 pm
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.mikesbytes wrote:Regardless whether its good logic or rubbish, I don't understand how the logic translates into calculating the speed limit.
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.
- Strawburger
- Posts: 1729
- Joined: Tue Aug 26, 2008 12:19 pm
- Location: Dulwich Hill, Sydney
Re: Speed Limits - 85 percentile
Postby Strawburger » Wed Oct 17, 2012 6:22 pm
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.
-
- Posts: 375
- Joined: Mon Aug 22, 2011 7:02 pm
Re: Speed Limits - 85 percentile
Postby tubby74 » Wed Oct 17, 2012 9:43 pm
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.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?
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
-
- Posts: 3056
- Joined: Tue Jan 05, 2010 2:54 pm
Re: Speed Limits - 85 percentile
Postby zero » Wed Oct 17, 2012 10:37 pm
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.tubby74 wrote: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.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?
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
85th percentile of Anzac bridge is probably 40km/hr ish I would think. No chance of it ever getting over 70.
- Howzat
- Posts: 850
- Joined: Wed Aug 15, 2012 7:08 pm
Re: Speed Limits - 85 percentile
Postby Howzat » Wed Oct 17, 2012 11:02 pm
Average vehicle speed on the main city-bound roads in the morning in Sydney is about 30 km/h.tubby74 wrote:85th percentile of Anzac bridge is probably 40km/hr ish I would think. No chance of it ever getting over 70.
I'm guessing 85th percentile is about 31 km/h.
- Strawburger
- Posts: 1729
- Joined: Tue Aug 26, 2008 12:19 pm
- Location: Dulwich Hill, Sydney
Re: Speed Limits - 85 percentile
Postby Strawburger » Thu Oct 18, 2012 4:50 am
Incidentally, the speed lowered due to altering the layout to attempt to fix congestion, thus lowering the safety standards.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
- mikesbytes
- Super Mod
- Posts: 22183
- Joined: Thu Dec 01, 2005 11:42 pm
- Location: Tempe, Sydney
- Contact:
Re: Speed Limits - 85 percentile
Postby mikesbytes » Thu Oct 18, 2012 8:58 am
- find_bruce
- Moderator
- Posts: 10615
- Joined: Mon May 09, 2011 8:42 pm
- Location: Sydney
Re: Speed Limits - 85 percentile
Postby find_bruce » Thu Oct 18, 2012 9:52 am
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.
- Strawburger
- Posts: 1729
- Joined: Tue Aug 26, 2008 12:19 pm
- Location: Dulwich Hill, Sydney
Re: Speed Limits - 85 percentile
Postby Strawburger » Thu Oct 18, 2012 7:26 pm
Going from memory here, I will look it up when i am back at work.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?
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?
-
- Posts: 375
- Joined: Mon Aug 22, 2011 7:02 pm
Re: Speed Limits - 85 percentile
Postby tubby74 » Thu Oct 18, 2012 9:26 pm
- mikesbytes
- Super Mod
- Posts: 22183
- Joined: Thu Dec 01, 2005 11:42 pm
- Location: Tempe, Sydney
- Contact:
Re: Speed Limits - 85 percentile
Postby mikesbytes » Tue Oct 23, 2012 11:35 am
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 calcuationStrawburger wrote:Going from memory here, I will look it up when i am back at work.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?
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?
Return to “Cycling Safety and Advocacy”
- General Australian Cycling Topics
- Info / announcements
- Buying a bike / parts
- General Cycling Discussion
- The Bike Shed
- Cycling Health
- Cycling Safety and Advocacy
- Women's Cycling
- Bike & Gear Reviews
- Cycling Trade
- Stolen Bikes
- Bicycle FAQs
- The Market Place
- Member to Member Bike and Gear Sales
- Want to Buy, Group Buy, Swap
- My Bikes or Gear Elsewhere
- Serious Biking
- Audax / Randonneuring
- Retro biking
- Commuting
- MTB
- Recumbents
- Fixed Gear/ Single Speed
- Track
- Electric Bicycles
- Cyclocross and Gravel Grinding
- Dragsters / Lowriders / Cruisers
- Children's Bikes
- Cargo Bikes and Utility Cycling
- Road Racing
- Road Biking
- Training
- Time Trial
- Triathlon
- International and National Tours and Events
- Cycle Touring
- Touring Australia
- Touring Overseas
- Touring Bikes and Equipment
- Australia
- Western Australia
- New South Wales
- Queensland
- South Australia
- Victoria
- ACT
- Tasmania
- Northern Territory
- Country & Regional
Who is online
Users browsing this forum: No registered users
- All times are UTC+10:00
- Top
- Delete cookies
About the Australian Cycling Forums
The Australian Cycling Forums is a welcoming community where you can ask questions and talk about the type of bikes and cycling topics you like.
Bicycles Network Australia
Forum Information
Connect with BNA
This website uses affiliate links to retail platforms including ebay, amazon, proviz and ribble.