50mph Speed Limit Would Cut CO2 By 30%, Says New Study

Reducing the speed limit to 50 mph, say researchers, would create the tipping point where taking transit or modes other than driving will become more attractive and therefore save CO2 emissions.

Also, it turns out that driving slower reduces emissions.

From Wired: "The faster you go, the more fuel you burn because wind resistance increases exponentially. Lowering your speed by 5 mph when traveling at 35 to 45 mph will boost fuel economy as much as 10 percent (.pdf), according to a report the General Accounting Office prepared in 2008."

Treehugger takes the Wired article a step further, presenting six reasons why slowing down is beneficial. For one:

"It could save a lot of fuel. Some estimates indicate up to 5%; In 1983, by which time many people were ignoring it, it saved 2.5 billion gallons of gasoline and diesel, or 2.2% of the total fuel used. Oil was a horrible $ 20 per barrel. Wired notes that "The Department of Energy noted in 2008 that lowering the national speed limit to 55 mph would save 175,000 to 275,000 barrels of oil daily."

Full Story: Slow Down and Spare the Planet



What good would putting up a

What good would putting up a speed limit that is lower, when the road was engineered for speeds much higher than the posted limit. For example, the sign may say 30 mph (50 km/h) but the road is built for 40 mph (60 km/h). Lower the limit does nothing if the road is unchanged.

Narrow the lanes, sharper turns, bike lanes, etc. are needed to help reduce speeds.

Speed limit.

You'd have to enforce speeds. For those opposed to hiring humans for labor, speeding camera technology has existed for ~two decades + . Speed reduction worked in the US in the past.



First Step To Lower Speeds

Maybe a politically feasible first step toward lower speed limits on freeways would be to designate one lane as a "green lane" with a speed limit of 50 miles per hour, to give people a choice of driving more slowly if they want to reduce their ghg emissions.

This would be self-enforcing, eliminating the worries in the other two comments, because if some of the people in the lane drove at 50 mph, everyone in the lane would have to drive as slowly.

It would also slow speeds in the other lanes a bit, assuming that relatively few people would drive at 50 mph, so the other lanes would become a bit more congested.

If there were publicity about the environmental benefits of driving in the green lanes, enough people might slow down for it to work.

Charles Siegel


Only those that seriously want to punish drivers for their sins could take something like this seriously. Plus, this idea has the added bonus that it has already been tried in the US and found not work (what's the definition of insanity again?). Installing speed cameras will not solve anything as they will either be shot out or disconnected (England has a very large problem with this as they have speed cameras all over the county), or create more problems as drivers will slam on the brakes at known camera locations, then speed up again once past.

For whatever reason some people relish then thought of punishing drivers to get them to take transit. However, driving speeds in most dense urban areas of the US (where transit is truly, or could truly be, a viable option) are already below the proposed limit during commute times, so this measure would do little to effect a shift to transit, but a lot to make everybody real angry (having to drive 50 down an interstate desinged for travel speeds around 90 mph when no one else around is simply absurd to anyone else but the head bureaucrat in charge of the rule). Plus, for any benefit of reduced emissions there are going to be costs not mentioned that will outweight the benefit (loss of time being the most important, but also increased roadway deaths - there are some studies on this from when the speed limit was 55, teaching a general disrespect for the law - hard to respect laws when people know they are arbitrary and capricious).

The way to get people to drive less is to stop subsidizing their behavior (as we know driving is subsidized). The article at least mentions it at the end.

Variable speed limits and automatic enforcement

The M-25 motorway in the UK has had variable speed-limits combined with automatic camera-enforcement for some time now (http://en.wikipedia.org/wiki/Speed_limit#Variable_speed_limits). As congestion goes up, the speed limit goes down so that the vehicle flow remains 'fluid'. They have gantries which indicate the speed limit in effect at any given moment (80/70/60/50 kilometres per hour!) and cameras which record the licence plates of each vehicle passing. If you go too fast, you will get a ticket in the mail. I believe that they also process the time *between* the gantries, so that they can calculate if you were going too fast at any point between them!

I heard a story about someone who had rented a car at Heathrow (the airport) and ignored all of the posted speed signs on the way into London: "I was going 100 the whole way and nobody stopped me." Upon returning the rental car he was charged with a few thousand pounds in speeding tickets!

Setting arbitrary speed limits which are not backed up by any kind of enforcement mechanism will clearly not work. And setting speed limits which have no context for the local situation is also a non-starter. On the other hand, lowering the speed limit in order to keep the traffic flowing is something which many people might support and it would reduce emissions as well.

But there is no substitute for proper pricing.

Local Context

It would be interesting to see a study on the M-25 to see if it really works at "keeping traffic flowing" any better than other congestion management schemes (metering lights, variable toll pricing, etc.). The likely answer is that it doesn't, but it sure does produce a lot of revenue for the authorities... it's the same reason every hick town on a rural route in the US posts the speed limit sign (notifying you of a drop in the speed limit) behind a conveniently overgrown bush. I say it's the likely answer because if it were wildly successful everybody would be copying it straight away.

You are correct that eveything has a local context, which is why the 55 mph speed limit (or the proposed 50 mph speed limit) is simply as stupid as stupid gets.

Charles, I hope you're kidding

Creating a Green lane with significantly slower traffic is a recipe for disaster unless it were physically divided from the other lanes. There's probably more unsafe things you can do than to have significantly varying speeds on the same highway, but not too many. How many accidents would be caused by people trying to pass in the Green lane and rear ending somebody? Did you really think that through? You really don't think that's viable do you?

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