May 13, 2013

Speed Limit on the Autobahn? - Nein!

Germany, that generally level-headed country, the one that produces some of the world's best engineers and engineered products, some of the most penetrating philosophers, some of the most forward-thinking policies on protecting the planet, went into a tail-spin at the weekend over the mere idea of imposing a speed limit on its Autobahn.

Photo by Q-ß

The hapless politician who had dared to air the idea is Sigmar Gabriel, the chairman of the opposition Social Democratic party (SPD).

Remember, this is the country that is phasing out all nuclear energy by 2022. Part of their "Energiewende" (energy transition) plan is to source 35% of their electricity from renewable sources by 2020, ramping to 100% by 2050. Heady stuff: these people are serious about clean energy.

The German Green Party has long been a proponent of a national speed limit, for reasons of energy conservation. And the similarly environmentally friendly Traffic Club of Germany (VCD) quotes a statistic that those German highways without speed limits are the location of 70% of fatal road accidents. But inside his own party, Mr. Gabriel finds no such support: Within hours of his statement other SPD members were careful to distance themselves from him.

Newspapers were less circumspect: an article in Die Welt was as polite as it comes, and still its title was "Sigmar Gabriels Frontalcrash mit dem Tempolimit". (Generally, journalists went wild with the puns). In the south of the country, home of BMW, beer fests and other industries that profit from the less-than-rational parts of the human psyche, the Süddeutsche daily has enough perspective to admit that the discussion over speed limits is not rational.

Its piece, titled "Irrationalities of the speed limit debate", starts out by declaring that "Americans insist on the weapons they carry; Germans on the ones they sit in", and compares the American slogan "Land of the Free" to that of ADAC, the German automobile association: "Freie Fahrt für freie Bürger" (Freedom to drive for free citizens).

Strong words. Mind, this wasn't even a parliamentary proposal: Mr. Gabriel had merely said in an interview that such a speed limit would be "meaningful", I suppose in the effort to conserve energy. And we're talking about a speed limit of probably 130 kph (about 80 mph), which is what it is on most German highways already.

From an energy standpoint, a speed limit makes sense because the air drag on a car grows like its velocity raised to the power three. A speed of 200 kph is 54% higher than 130 kph, but takes nearly four times as much power to overcome the air resistance.

But the real problem with an absense of speed limit is the differential speed: if you're going 220 kph but trucks on the same highway can only go 120 kph, you're passing them, one lane over, at a differential speed of 100 kph (or 62 mph); read about my white-knuckle high-speed experience in the post where I ponder the question of how much horsepower a car really needs.

If you can cut the number of road fatalities by 70%, and conserve serious energy at the same time, I would call that a win-win situation, rationally speaking. Of course, I'm a chicken as well as a physics geek: going 220 kph give me no exhilaration, it only makes me start to compute conservation-of-momentum equations in my head. In the case of your fast passenger car colliding with a large, heavy and slow truck, the truck will always win.



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  1. WOW! I am so not a physics geek - but what you say really makes sense. I have never thought about it! I am also not a risk taker - and will gladly relent to any slow truck.

    I prefer to live. If you save energy while doing it - I also think it's a win-win.

    Love this post!

    1. It appears we each have our blind spot...
      Thanks for visiting!


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