Monday, May 15, 2006

Slow down & save oil

Francis Slakey in the NewScientist this week has a suggestion for resolving the US energy crisis – get people to drive slower by lowering national speed limits to 55 miles per hour.
‘ Every car engine has a sweet spot in terms of efficiency, typically when running at 55 mph. Beyond 60 mph, engine efficiency plummets because of higher temperatures. In addition, three other factors that affect gasoline consumption become more significant at higher speeds: tire resistance, wheel bearing friction and air drag. What all that means is that for the average car, cutting the speed from 75 mph to 55 mph improves fuel efficiency by roughly 25%.
This would save 1 billion barrels of oil per year - more than US imports from the Persian Gulf. Former President Carter endorsed a 55 mph limit in the oil crisis of the 1970s but the public kicked him out. Lowering the speed limit did reduce demand and help cut gasoline prices. But there are some arguments for again trying such a policy:

  • Reducing the limit to 55 mph just broadens the sense of public good to include conservation.
  • For most people a reduced limit would not add greatly to travel times and those who it would harm – e.g. long-distance truckers - would spend less money on gasoline - they would save more than 50 cents per gallon at current prices.
  • There would be fewer serious traffic accidents.

A flaw in the plan is that consumers alone pay. President Carter forced US car makers to establish an average fuel efficiency standard - the Corporate Average Fuel Efficiency (CAFE) standard - across their fleet of 27.5 miles per gallon within a decade. This has been fixed since though it too could be tightened to 33 miles per gallon today – higher targets than this have been suggested by the Sierra Club. With this change the US would save a further 2 million barrels of oil a day and there would be incentives too for car manufacturers to improve an engine's sweet spot to make it run more efficiently at 75 mph.

The Slakey plan is worth examining. The cost savings in fuel bills and the reduction in traffic damage accident costs need to be assessed against the value of increased travel time valued at some function of real wages. A good thesis topic for an economics student once the core science is checked out – my reservation is only that fuel economy in vehicles I drive is better on long-distance trips when I drive faster than when I drive around town - due I assume to more stop-and-start driving in urban areas. I have always believed (falsely?) that increased fuel consumption was driven by the need to accelerate a vehicle but that it would be low if one is travelling at a relatively high speed but not accelerating.

Of course, if the argument is correct, one can individually reduce fuel bills by slowing down! The advantage of a law is the macroeconomic effects of induced lower oil prices.


Patrick said...

I assume that you are aware that you are making an argument for raising most speed limits in Australia?

And by even more than implied, since big Foldens often have a 'sweet spot' more like about 65-70mph :)

hc said...

Patrick, It depends what you mean by most - by distance or by usage.

The speed limits on most highways and expressways in Australia are 100-110 km/h and this exceeds the recommended 55 mph limit which corresponds to 88.5 km/h.

I assume most urban travel would satisfy this speed condition as you suggest.

Australia is a highly urbanised society so maybe the gains would be less here as the speed constraint would not bind so frequently here.

How do you know about the 'sweet spots' of large Fords and Holdens? I thought larger vehicles would be as common in the US as here so this should be reflected in the Slakey computation.

mlesich said...

Professor Clarke

If it is such a good idea to slow down in order to save petrol, why do we need the government to do it for us? Now I know that the government of the day has a role to correct market failures such as those that arise out of the consumption of petrol but couldn't this externality be corrected through a tax? What I want to know is are there any other failures that prevent the most efficient outcome from being reached


The Crackpot Libertarian

hc said...

hi mlesich. Its a good point - economists generally like prices and taxes not regulations.

Maybe people can be induced to slow down simply by informing them of the cost savings they will enjoy. If they can this discussion may be unnecessary.

Or is it that individuals will still drive too fast from a social viewpoint because, while they identify the private savings they will enjoy, they fail in the US anyway to appreciate the higher aggregate savings from induced effects of demand reductions on the price of oil.

Also I am not sure that you can determine your car's travel speed? Most people in my experience drive at close to the legal limit and individual 'slow coaches' are going to find the going difficult and perhaps dangerous. Is it bpurely an issue that motorists don't understand the cost implications of driving fast?

You don't sound like a Crackpot Libertarian at all. When I was writing this post I was trying to think of market signals you might want to send out that would emphasise the dependence of cost on speed. All I could think was to provide information (as above) or impose limits (as per he post).

Sam Ward said...

"How do you know about the 'sweet spots' of large Fords and Holdens? I thought larger vehicles would be as common in the US as here so this should be reflected in the Slakey computation.

Cars may be bigger in size terms in the US but Australia's massively overpowered straight-6 family sedans are fairly unique and are designed with high speed long-distance cruising in mind. A cruising speed of 110-120 km/hr is the "sweet spot" for a falcon or commodore - you don't even need to know anything about cars to know this, you can hear it in the engine or transmission when you are on the highway.

For smaller, 4 cyclinder vehicles, the 90 km/hr mark seems about right.

Jan said...

Informing people is as far as I would go. Imposing limits or tax would, in addition to the large bureaucratic and logistics costs, constitute significant indirect cost on people to adapt (which would in the transition period lead to more, not fewer car accidents). More importantly, this should be an individual decision since it depends on the opportunity cost of one’s time. Do the maths: Slowing down on the freeway from 110 to 90 km/h would roughly lead to a saving of about 0.5-2 liters of petrol for a normal sized car. For many people, even if they did internalize the positive externality of lower oil prices, the saving would still be below their opportunity cost of those 20 minutes. Why should they have to do this? And what happens if oil prices go down, do we change the limits/tax again? Or do we make the limits a function of the oil price?...

Patrick said...

I assumed that the post was a joke. The idea of making speed limits on the basis of fuel conservation is not 'flawed', it is fundamentally wrong.

a) marrying a given engine's 'sweet spot' to a particular speed is a tricky exercise at the best of times: how is this car geared? How heavy is it? What surface is it on?

b) with modern luxury cars, and thus with the cars of the near future, sophisticated injection and timing systems and 'infinite' transmissions mean that you have a 'sweet range', and it is probably enormous.

c) all that is irrelevant, because even if every car had a sweet spot of precisely 55mph it would still be irrelevant to any sane policy maker, because:

- it has no useful contribution to make poicy-wise, because speed limits are concerned mainly with safety, which suggests much less than 55mph in residential areas and much more on long straight roads (so the driver doesn't fall asleep inbetween Melbourne and Geelong).

- it has no useful contribution to make even if we cared about conservation over safety because probably scarcely 10% of car driving is in 'optimal' conditions: people drive in the city where you don't want 55 mph anyway, they are constantly accelerating and braking and stopping and starting, etc. People in 'optimal' conditions should be going much faster than 55mph normally, since they are probably in bigger cars that are quite comfortable at those higher speeds and because driving any distance at that speed threatens to induce hysteria or catatonia, depending on the personality.

If you want to use less fuel, drive barefoot, anticipate stopping and coast instead of braking, and pay attention to how you corner.

hc said...

Patrick, It wasn't a joke (as far as I know) although it may have been mainly suggested for US conditions. And I assume for expressways, highways not high density urban travel where the speed limit would be much lower anyway.

As I stated in the post I also believed fuel consumption was related to acceleration rather than cruising at a sustained speed. If this is so then physicist Slakey is off mark.

Robert Merkel said...

Here's a very, very rough analysis in the context of a single scenario.

In most cars, in most conditions, a freeway cruise at 100 km/h is going to be more economical than one at 120 km/h - at a rough guess the increased speed increases fuel usage about 20%. That's trivially Googleable.

Typically a Falcadore will get about 7.5 litres per 100 kilometres at regular cruising speeds, so the difference is about 1.5 litres, or a bit over $2 worth of petrol at current prices.

The car travelling at 120 km/h will cover 100 kilometres in 50 minutes. Therefore, the extra speed saves you 10 minutes of time.

On this basis, it costs $12 worth of fuel per hour of travel time saved, not including the marginal extra wear on the car, and any externalities.

I'm not sure, but I assume for most people an hour of their time is worth considerably more than $12.

If you want to give a student something to really chew on, do a cost-benefit analysis of the compulsory fitment of tyre pressure monitoring devices to all new cars.

Patrick said...

20% sounds so improbable as to not even be worth googling!

Not to mention that even if you want to focus on highways (which I suspected you would) it remains an irrelevant consideration because of the amount of vehicles on highways using very different diesel engines and using LPG. Ulless you want to assign lanes: truck diesel, small diesel, LPG, 8 cylinder, 6 cylinder, 4 cylinder, modern automatics to use any.

What is worth googling is that cars contribute to about 8% of CO2 emissions in Australia.

PS Harry, being practical, as I try to, I meant above actual distance used.

Anonymous said...

As a student of physics, your solution is quite obvious.
I am curious however. Do you have any information availible as to the current energy savings for the United States if we reinstated the 55 mph speed limit?

I am trying my best to find reliable figures, but they seem "strangely" elusive.

Thank you for your time and effort,


TruthSeeker said...

There is a lot of wishful thinking going on in these comments. The truth is that larger, heavier vehicles have the lowest sweet spots, not vice versa:
Quoted from:

So, for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph. Cars with a higher road load will reach the sweet spot at a lower speed. Some of the main factors that determine the road load of the car are:
•Coefficient of drag. This is an indicator of how aerodynamic a car is due only to its shape. The most aerodynamic cars today have a drag coefficient that is about half that of some pickups and SUVs.
•Frontal area. This depends mostly on the size of the car. Big SUVs have more than double the frontal area of some small cars.
•Weight. This affects the amount of drag the tires put on the car. Big SUVs can weigh two to three times what the smallest cars weigh.
In general, smaller, lighter, more aerodynamic cars will get their best mileage at higher speeds. Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds.
If you drive your car in the "sweet spot" you will get the best possible mileage for that car. [end quote]