Appropriate speed

Road conditions are constantly changing, so our current system of speed limits provides a blunt approach to traffic control, that is rarely appropriate. Australia-based road safety research manager John Lambert says it is time to move beyond the group-think of speed limit zealots, and consider how appropriate speed limits might be adopted.

In any road situation there is a travel speed that is appropriate and includes a safety margin, and there is a higher threshold speed below which a crash is most unlikely and above which a crash rapidly becomes inevitable.

For example on a curve on a dry day there is a maximum speed above which the grip of your tyres will be insufficient to allow you to get around that curve. And typically the threshold speed in good conditions is 80% to 100% greater than a responsible driver would chose to travel around that curve.

Constant change

The appropriate travel speed is constantly changing depending on whether the road is dry, wet or icy, whether the weather is clear, rainy or foggy, the sight distance in relation to potential collisions. The appropriate speed on the same section of road is much lower outside a school with cars parked both sides of the roads and children crossing the road in drop off and pick up times than at the weekend when there are no parked cars or children so sight distances are much longer. Other important factors include the road geometry (pavement width, curvature, divided or undivided, width of the road shoulder, clearance distance to fixed objects et cetera), the characteristics of the vehicle you’re driving (a truck with a high load must negotiate a curve at a lower speed than a car), and so forth.

Speed limits are a hopeless indicator of the appropriate travel speed and are not even set based on the appropriate speed for the type of road or area – if they were the speed limit on motorways (which have by far the lowest crash rate per 100 million kilometers) would be significantly higher. And along any type of road, or within any area, the appropriate speed varies from being higher than the speed limit, to much lower than the speed limit.

Matter of choice

Speed related risk is primarily determined by a) the appropriate speed, which most drivers are very good at estimating, and b) the speed individual drivers choose to travel at in relation to that appropriate speed.

Travelling 6mph above that appropriate speed (which is not determined by the speed limit) in an urban environment increases crash risk by around eight times; 10 mph above that appropriate speed by 30 times; and 10 mph below that appropriate speed (unless you have slow vehicle warning lights and/or signs) increases crash risk four times.

And the outcome of a crash is determined by the impact speed, not the travel speed! The data on pedestrian crashes in UK 30mph zones suggests / proves the impact speeds are well below 10 mph (otherwise the ratio of deaths to injuries would be much higher). And the appropriate travel speed is one that on almost all occasions allows the driver to stop before an impact or swerve to avoid an impact.

And responsible drivers are very good at judging the appropriate speed – in spite of facing many potential crash situations each day, the chance a responsible driver will not be in a crash where someone is injured to the degree they require a doctor is once in five life times. And the chance they will be in a crash where someone dies is around less than once in 20,000 years!

Inappropriate speeds

It’s not drivers exceeding the speed limit when appropriate that poses a hazard to the more vulnerable road user groups. It’s those travelling at inappropriate speeds whether that is above or below the speed limit.

And Intelligent Speed Adaption is an absolutely stupid idea that is not worthy of consideration because speed limits are not the appropriate speeds.

The challenge is to get the road safety zealots to abandon their collective group think, and alter the legislation and the enforcement to penalise those travelling at inappropriate speeds for the situation.

Role for speed cameras

Speed camera technology could be adapted to achieve this. The camera would continually monitor average speeds at the location and then photograph those vehicles travelling 6 mph or more above that speed or 10 mph below that appropriate speed (which assumes the majority of drivers get the speed right). Then for slower vehicles inspect the photos to see whether it has relevant lights and or signs, and if they do cancel the infringement. And penalise the others!

Note: I have been involved in road safety for 43 years, have managed road safety research in Victoria, Australia for four years, and have been involved in speed, and speed and crash research for 22 years.
 

Your basic premise and philosophy are quite correct. While we generally are not in favor of speed cameras at this point, our thoughts are to use them in freeway work areas, measuring the 85th percentile speed and "zapping" those vehicles exceeding that value by 10 mph - constantly throughout the day - with lots of PR!
 
With the correct publicity, we envision a reduction in the travel speed by the faster vehicles, spilling over to the conventional road system without the benefit of cameras.
 
The public should become quite aware of "orange WZ" signs along our freeway system.
 
Education and "instinctive" driver behavioral adjustment are the goals!