Credit: veer
For many of us, driving a car becomes routine. We buckle ourselves in and turn the key to ignite the engine; check our mirrors, shift into gear and seamlessly pull away into traffic.
But sometimes on the road we cheat, speeding just a few kilometres per hour over the designated limit in an effort to shave a few minutes off our travel time.
Travelling 65 km/hour in a sixty zone might seem like a relatively minor infraction, and maybe won’t result in any speeding tickets, but it could factor into your likelihood of being involved in a collision – if the appropriate hazard presents itself.
In a February 2011 study, researchers from the University of Adelaide concluded that the greatest prospective gain for reducing the number of car crash injuries would come from targeting low level speeders – those going just one to five kilometres per hour above the limit.
Using statewide surveys, they observed vehicle speeds at 127 different sites in South Australia, along urban and rural roads where speed limits ranged from 50 to 110 km/hour; and they analysed crash sites where injuries or fatalities occurred.
While any speed reduction would likely reduce the incidence of injury and death, they found that that the greatest overall benefit for preventing casualty crashes “will come from reducing speeds just above the speed limit in almost all cases.”
The findings build on a 2001 report published by the same group at the university’s Centre for Automotive Safety Research. Using data from road crashes, they determined that the risk of being involved in a dangerous collision doubled with each five kilometer per hour speed increase above 60 km/hour.
Therefore, a car driving at 65 km/hour would be twice as likely to be involved in a crash resulting in death or hospitalisation of one of the involved parties, as a car abiding by the speed limit. A car racing at 70 km/hour would see the risk quadruple.
“Unfortunately the risk of driving only a small amount over the speed limit is not intuitive to most drivers, as they have not encountered any problems with doing so,” says study author Sam Doecke, a crash investigator and automotive engineer at the University of Adelaide.
An individual’s crash risk is a very small number, explains Doecke, so when it doubles as a result of driving 5 km/hour faster than the speed limit, it still seems insignificant.
“However, if all the millions of drivers in Australia did this all the time we would expect injury crashes to double,” he says.
It may seem insignificant, but hovering just a few km per hour above the speed limit could potentially be the difference between a safe stop and a costly collision.
The first thing to consider here is the human response time – this includes the fraction of a second it takes the driver to perceive a hazard on the road, such as a cyclist or animal, and then the amount of time it takes to react and apply the brakes.
The Error of Looking at Accident Numbers as a Physics Problem.
The issue with proposing a high level of enforcement on minor speed limit infractions is that despite the physics saying it could reduce accident numbers and damage is that it fails to account for social and Psychological factors. Rigid enforcement would likely have drivers looking at their speed all the time rather than the road, traffic and potential hazards. If this is the case then accident levels and corresponding injuries would likely increase rather than decrease.
Then there is the legal issues. No mass produced car has an accurate speedo, certainly not one accurate to within a few kilometres, nor by law do they have to as safety and vehicle standards have an error tolerance built in. Even if they were 100% accurate when a vehicle was new it would progressively become less accurate as the tyres wear. I could imagine he court system collapsing under the load of disputed infringement notices.
I think a better way to attack the issue is to enforce vehicle separation, appropriate lane usage and associated infringements for driving too slow as disparate speeds are also a major contributor to accidents.
The study is an interesting exercise and confirms or validates information that was already known or assumed, however to say enforcing speed limits to such a low level is simplistic and unenforceable. Better to use the information to push for compulsory heads up displays, anti collision radars and embedded guidance technology to take the task away from inattentive humans
Re: Enforcement on Minor Speed Limit Infractions
I would agree with your comment that it would be nearly impossible to lawfully enforce the limit to within 5km/h, but wouldn't something like an advertising campaign using some of the numbers from studies like these be able to prevent drivers from taking the risk of being "5 k's over" whilst avoiding the paranoia that would come from the enforcement of it.
Auto Accidents Physics Problem
After a time in the Highway Patrol, I must agree absolutely, with the reader who proposed aggressively seeking safe distances between drivers. High speed alone is not the determining factor, but may applicable where vision is blocked by curves and hills.
On highways, however, distance between cars, is the prime consideration, and appears to be overlooked by most police. It requires more skill than simply using a radar gun, and is more easily disputed, but it should be penalized.
One last addition, due to the limited vision attendant to night driving, speeds should be lower at night.
Speed or Gap?
While there is no doubt in the merit of the article (slower speeds = reduced impacts in the event of a collision = less injuries & deaths on the roads) one very important factor in the cause of road collisions was brushed over as a mere inconvenience in the calculations … drivers reaction time – or more specifically; drivers reaction time & the relationship between that to the gap maintained to the car in front.
Using the same simple scenario cited in the article of a car travelling at 50kph, with the typical reaction time of 1.5 seconds the car will travel 21 meters between when the hazard is recognised and when the brakes are applied - agreed.
Now let’s add in the distance given to the car in front. Using the two-second gap method prescribed by the RTA in NSW (longer in adverse conditions) things are generally ok as the car behind is 28 meters behind the one in front. In this situation, the driver recognises the brake lights in front, applies the brakes themselves all with a ‘healthy’ half second or 7 meters to spare – sadly too few drivers leave such a gap.
In the real world (just like many drivers habitually speed a few kph over the limit) many drivers habitually leave dangerously small gaps to the cars in front – on any given day you can observe drivers routinely travelling barely 5 meters (approx. one car-length) or even less behind the car in front, even at over 100kph on the motorway!
In the example provided; at 50kph with the assumed deceleration of 10m/s squared pulls the car up in 9.6m from the moment the brakes are applied - agreed. For a car travelling behind leaving just a 5 meter gap to the car in front, this is a real problem. The car in front is about to come to a complete stop just 14.6 meters up the road from where he or she is when those brake lights come on – as already established the car behind is going to travel 21 meters (over 6 meters past the rear of the car in front) before they even apply the brakes themselves!
So this begs the question, why don’t we see more collisions on our roads? It’s so common to see gaps between cars no longer than a car length, why don’t we see more nose-to-tail incidents of this nature? The answer is because we rarely employ maximum braking of this nature to stop a car – however on occasions there is a need to engage emergency braking, when that’s combined with the car behind not giving sufficient gap for them to stop, a collision is inevitable no matter what speed they are travelling.
To underline the importance of the gap size over the vehicle speed, let’s use an example where a driver is going 5kph less than the speed limit in a 60kph zone. This article suggests that he or she would be significantly safer than if they were doing the 60kph limit, correct? Well not really, if that same drivers was following the car in front by 10 meters (or more than two car lengths) even though they are moving less than the speed limit at only 55kph, they still carry a velocity of a little over 15m/s, this means the driver would need a reaction time of less than two thirds of a second to avoid a collision with the car in front in an emergency braking situation – not even the best drivers in the world are capable of this.
Reducing speed on the roads is a good thing, but increasing the gap to the car in front is even better. Reducing the gap to the ridiculously small distances we commonly see on our roads presents a far greater risk of creating, and increasing the severity of, a collision resulting in injuries and/or fatalities than going several kph over the magical number painted on a sign we call the speed limit ever does.