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The deadly physics of road crashes

By Joseph Froncioni, Former Chairman, Bermuda Road Safety Council

According to an article that appeared in this newspaper on December 28, 2012, as of the end of November 2012, traffic collisions were down about six percent compared to the same period in 2011 and crashes causing injury were down just over three percent. Looked like good news to me until I continued reading. The article went on to say that the number of people who were seriously hurt went up 15 percent and the number very seriously hurt and requiring admission to the ICU went up 64 percent. That is very bad news and cause for alarm in my opinion. Let me explain.

The results of the study my co-authors and I did looking at 3,673 road traffic injuries from 2003 and 2004 showed a 67 percent increase in road injuries among residents compared to a similar study carried out in 1993. While the probable causes for this dramatic increase are numerous, one of the most important is the steady increase in travelling speeds on our roads.

At the time, we postulated that since Commissioner Colin Coxall all but disbanded the motorcycle traffic division in 1996 in response to recommendations made in the Tumin Report, the lack of police presence on the roads resulted in a slow but steady erosion of safe driving habits in Bermuda. This trend has continued. I will not bore you with a litany of road traffic infractions that one sees on even the shortest of trips nowadays but rather ask you to hear me out on what I believe is surely one of the most significant, ie speeding.

Hardly anyone would disagree that over the last 15 years or so, the average speed in Bermuda has steadily climbed. In the world of road safety, we speak of speed creep. In a nutshell, “When the cat’s away, the mice will play”. Take the police off the roads and the average speed will gradually increase. By my estimate, accepting that it is very difficult to be accurate about these numbers, the average speed in Bermuda has gone from 45kph in the early 1990s to 55kph today, an increase of about 20 percent.

While this may not be news to you, you were probably unaware of the inevitable consequences of such an increase and that is that case-fatality rate, the proportion of those sustaining injuries who die of those injuries, varies to the Newtonian fourth power of the velocity at vehicular impact. Stated another way, small increases in average speed lead to higher speeds at impact resulting in very large increases in severe injury and death on the road.

The connection between travel speed and injury severity has been well studied. We know that the dissipation of energy that necessarily occurs in a collision is expressed by the physical relationship between the vehicle mass and its speed. Kinetic energy is generated by the moving vehicle as a function of the square of the speed rather than the speed itself. For those who are interested, this simple equation applies: Kinetic Energy = ½ x mass x (velocity) 2.

In our example, a 20 percent increase in speed yields a 44 percent increase in kinetic energy. That kinetic energy is dissipated at impact as sound, heat, twisted metal and, tragically, twisted limbs. Human bodies are robust but unfortunately, when the forces of deceleration and shear surpass a certain threshold, the physical bonds that hold our tissues and organs together are broken and injury and death results.

As if the situation was not bad enough, physics has further conspired to make things much, much worse for us in Bermuda. Here’s the second problem, first in scientific doublespeak.

When vehicles of unequal mass crash into each other, the ratio of driver fatality risk in the lighter vehicle to risk in the heavier vehicle (the fatality risk ratio) increases as a power function of the ratio of the mass of the heavier vehicle to that of the lighter vehicle (the mass ratio). This is because when two vehicles crash head-on into each other, the ratio of their changes in speed (delta-v) is inversely proportional to the mass ratio and we know that delta-v affects driver injury risk.

In English, our roads are undivided, predisposing to head-on collisions, and the vehicles on those roads are a mix of motorbikes and cars and trucks. Motorbikes are very much lighter than the other two types of vehicle and so when a motorbike collides with one of them, guess which one comes to a more sudden stop and which driver gets the more serious injuries. The motorbike driver loses nearly all of the time. Physics wins again.

So, it looks like we have seen a slight drop in the number of collisions but the severity of injuries has risen sharply. Speed kills, or at the very least, injures severely and the higher the speed, the greater the carnage. Does anyone think we should address this problem? I’ll help you come to an answer by telling you a bit about how the same problem was (or wasn’t) addressed by our cousins, the Brits and the Americans.

Between 1990 and 1999, road deaths dropped by 33.9 percent in the UK compared to 6.5 percent in the US. This sharp decrease in road deaths seen in the UK is felt to be attributable to the 29.6 percent drop in case-fatality rate and differs greatly from the 6.6 percent decrease in case-fatality rate seen in the US. The chart illustrates the difference well (see original article here: http://www.sciencedirect.com/science/article/pii/S0749379705003314).

In the early 1990s, the United Kingdom introduced speed cameras as well as an array of speed-calming measures. In contrast, in the United States, use of speed cameras was extremely rare, and speed limits and speeds increased in 32 of the 50 states, mostly in 1995 and 1996 when federal highway subsidies were unlinked from the mandatory 55mph speed limit by President Clinton. The reductions in case-fatality rates, probably from small drops in speed of impact, account for the disproportionately greater drop in death tolls in the United Kingdom compared to the United States. Note also the increase in case-fatality rate in the US after the mandatory 55mph speed limit was abandoned in 1995.

When I started putting this article together a couple of weeks ago, Bermuda had already seen two road deaths. As I write these last few words, the count has risen to five; that’s about one road death every six days so far this year. This begs the question: How many road deaths do we need for our politicians to summon up the courage to address the problem? Do our elected officials have the wisdom and fortitude to understand the magnitude of the social and economic burden these injuries and deaths are imposing on Bermuda? The problem is clear and the solutions are obvious. Speed cameras and a marked increased police presence on the roads would go a long way to solving our problem. It’s high time our leaders get down to work and do the right thing for the people of Bermuda.

* In an upcoming article, I will address the other important road safety issue, impaired driving.

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Published February 07, 2013 at 2:05 pm (Updated February 07, 2013 at 2:04 pm)

The deadly physics of road crashes

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