January 31


Strict Speed Limits – Ignoring the 85th Percentile

By Mike Spack

January 31, 2017

85th Percentile, Australia, Pedestrian Safety, speed limit, traffic safety

Guest Post by Bryant Ficek, PE, PTOE, Vice President at Spack Consulting.

We’ve previously examined the 85th percentile speed and how it is used in Minnesota and many other places to set the speed limits on roads. Essentially, we take a survey of existing speeds and set the road speed limit at the point 85 percent of drivers will drive at or below under free-flowing conditions. This method has many advantages including better compliance by the drivers without needing excessive enforcement. However, there are other ways to go about setting speed limits.

In Australia, two different “default” speed limits are used. These automatically apply everywhere unless overruled by a posted speed limit sign. The defaults are:

  • 50 km/h (31 mph) within built-up or urban areas
  • 100 km/h (62 mph) outside built-up areas or rural areas

In school areas, the posted speed is usually reduced further to 40 km/h (25 mph). In other pedestrian heavy areas, the speed limit is often posted at 10 km/h (6 mph).

These laws have been set based on a desire to improve safety for all users. Research has shown that:

  • A crash is less likely to occur speeds of 50 km/h (31 mph) as drivers have more reaction time
  • Pedestrians are significantly more likely to survive a crash at speeds below 50 km/h (31 mph)

These limits were introduced in 2001 on roads in the Australian state of Victoria and their research has shown fewer deaths since that time even as vehicle miles traveled has increased.

Ideally, all the roads would be designed with these speeds in mind meaning the desired 85th percentile speed of drivers would match the speed limits. In reality, Australia relies on heavy enforcement to compel drivers to obey the limits. Speed cameras, like red-light running cameras, are located along many corridors and do not require a police officer’s time to fine and put demerit points on a license. Radar and LIDAR are also widely used in addition to aircraft in some areas. The ability to have fixed and mobile camera sites in particular has the desired effect on slowing drivers to their speed. Engineering also gets involved where they can in the form of signal timing on corridors.

There are pros and cons to this type of approach, including privacy concerns with cameras (as we and others have learned in attempting to implement red-light running cameras). I don’t think Minnesota or the U.S. in general will go down this road. It was tried briefly in the 1970’s with a national speed limit of 55 mph on the interstates and quickly went away in the face of public opposition.

A better approach might be to decide the desired speed limit for a roadway based on function, land use, and other characteristics. Then during construction or reconstruction, add engineering elements to naturally enforce that design. This could include roundabouts at intersections, introducing horizontal or vertical curvature, using landscaping to limit sight lines, or many other measures under our control during this time. When a driver instinctively uses these characteristics, they won’t know we’ve done our job to reduce speeding without police enforcement.

  • I think this is going about it the wrong way. Roadway engineers should keep in mind the goal of getting road users to their destination as quickly as possible and use their budget to enable this. Speed limits are probably a bad idea, but if they must be set should be based on the capabilities of the roadway with respect to the best of modern vehicles and under ideal conditions, otherwise they unnecessarily restrict motorists. Last time I was in Australia there were not limits in much of the Northern Territory and good safety results were achieved. Enforcement can be based on actual reckless driving rather than a fixed speed as we can easily go over 150km/h without difficulty.

  • I’d be curious to know the research controls used with respect to the statement “These limits were introduced in 2001 on roads in the Australian state of Victoria and their research has shown fewer deaths since that time even as vehicle miles traveled has increased”. After all, in the US, highway speed limits have gone up since 1974 but deaths are down significantly since then, so one could argue with a similar statement that higher speed limits are safer. Correlation, of course, does not necessarily equal causation in either case.

  • Maximum travel speed is not the only objective to consider. The purpose of a road is to move persons from one location to another. Presumably they have activities at both ends of the trip, and often that activity is proximate to a roadway. The value of their experience depends upon not only travel time, but also other factors to consider such as comfort, sense of safety, aesthetic experience, pass-by capture for retail uses, and so forth. A high pedestrian urban setting, a commercial area having frequent starts and stops, a bus route, a scenic parkway, or a rural road with frequent horse drawn or other farm equipment, or frequent bicycle traffic, might justify lower than maximum speeds and a delayed travel time. It is the overall quality of life experience within context that matters, not merely shaving a few seconds or minutes from transit time.

  • I don’t know if you are aware of this or not . Fairly recently a researcher at the University of WA by the name of Dr Vanessa Bowden conducted some tests on a driving simulator to gauge performance of drivers under different types of speed limit enforcement . The simulated enforcements were :
    1) Driving in an area saturated with speed cameras and no tolerance at all for being over the speed
    limit .
    2) Driving in an area with speed cameras , but a tolerance for being up to 6kph over the limit .
    3 ) Driving in the same area with an 11kph tolerance .
    As a result of these tests it was shown that a driver in the first category was under stress and did not
    become aware of some potential traffic hazards , whereas a driver or drivers in the 2nd and 3rd
    categories were more relaxed, more alert and were better at identifying potential hazards .
    Now here comes the crunch. After these test results were published in the newspaper , Kim Papalia ,who is the Commissioner for Road safety in WA,was seen on a TV news footage totally rejecting these outcomes . What chance is there of improving road safety when we have high profile officials
    with narrow minded head in the sand attitudes .

  • Let me get this straight. If a speed limit is deemed by most to be too low for a road, your solution is to reduce sight lines and add obstacles to make the road as dangerous as the limit implies…… for Safety?

  • Not to mention a vehicles extra wear and tear from dealing with these obstacles on a daily basis… Which ultimately makes vehicles more dangerous and expensive over time.

  • I completely agree with your quality of life experience, where it is required but my disappointment lie’s with Australia’s highway and freeway underutilised potentional for higher posted speed limits. Plus the countries blatant revenue raising goals and the tactics the law enforcers use to grow revinue.

  • My guess is that if speed enforcement has been aggressive, such as in Ohio, that speeding tickets constitute a fair amount of local revenue, especially in smaller towns, and there would be a lot of resistance on the part of public officials to give-up some of their revenue stream in order to improve safety.

  • This will never happen as the states are too heavily reliant on the revenue from the drivers going a couple of kilometres per hour over the posted (often unrealistic) limit.

  • I’ve been looking for sometime unsuccessfully now to find the original reference for the 85th percentile theory. Do you have a URL for that reference or ideas on where to look? I think it came about during the 1960’s. As it relates to speed limits, I feel that traffic engineers tend to be focused on efficiently moving traffic (higher speeds are good) at the expense of considering the livability of the neighborhoods for residents, pedestrians and bicyclists (lower speeds are good).

  • This tactic is most often utilized in heavily pedestrianized areas where–for the safety of *other* road users–it’s imperative to slow driver speeds. Typically these changes involve improvements to the pedestrian environment. An example would be bumping the curb further into the road at intersections and crossings. The narrowing of the road both slows the driver to a safe speed for the pedestrian to judge a safe place to cross while giving the driver a better visual of the crossing pedestrian. Some tactics–like adding scaled street lighting or boulevard trees–don’t even change the physical layout of the road. They’re just visual cues. Traffic calming is likely not something you’ll see on motorways and expressways, but mainly urban streets.

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    Mike Spack

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