Saturday, 1 October 2011

And we go cruising to entertain ourselves




8 comments:

  1. I did a similar analysis a few years on why HSTs are a bad idea: same principle, that at running speed HSTs are dominated by that cubic air resistance (even more so for trains because of the much lower rolling resistance). The next biggest term, even for trains of limited stops, is KE from the station stops. (I think it was ECML I was thinking of).

    There's an extra argument for HST that it might replace other, worse forms of travel, but I find it hard to believe that eight times as many trains, of limited stops, travelling at 70mph wouldn't be a better idea than one running at 140mph (and opening more ways for 70mph trains along existing corridors more effective than a completely new ultra-low-grade smooth corridor at 140mph).

    But like you say, basic knowledge of the physical world has always been seen as gauche and faintly amusing to the Conservative party, like being foreign or female, so it doesn't surprise me that they're supporting high speed rail (or 80mph speed limits). I'm just really surprised that the other major parties seem to be in favour. I suppose the Lib Dems "like Trains" in a Sheldon Cooper kind of way, and the currently dominant Labour faction likes the idea of giant monuments to their own brilliance.

    Sorry, I've rather drifted. I just read through what you wrote, and it reminded me of all that work, years ago finding out the various friction co-efficients of lead, middle and trailing cars, etc, :-). I wish more people would do the kind of thing you've done. I didn't mean to, I just wanted to say, I agree!

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  2. If we're going entirely on numbers like these, perhaps we should look at reducing the speed limit, then? What would be the optimal speed in terms of total cost (both time and fuel)? If it works out to, say, 55mph, should we trust the numbers and reduce the speed limit to 55?

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  3. Really interesting read but predicated on the idea (as is the policy proposal) that these people are currently driving at 70mph. I'd suggest that they're already mostly driving at 80mph+ and an increase in the speed limit will probably bring a corresponding hike in their speed above 80.

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  4. Is slipstream from an equal-velocity vehicle in front going to make much difference to the air resistance calculations? (Assuming the vehicle behind isn’t tailgating.)

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  5. I got a bit suspicious of the 80mph thing once I realised that 1) most of the cost of fuel is, in fact, tax*, and 2) the treasury is very short of money.

    What I wonder is, given the increase in fuel consumption is, as you have figured out, quite a bit, but that not everyone will do 80mph all the time, will this raise enough money to have figured as a significant factor in govt calculations?


    (*I have no problem with this in itself by the way, it is a de facto carbon tax and therefore not necessarily a bad thing)

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  6. Don't think the numbers you quote are particularly relevant. This is an american article and so
    1. its in US gallons (approx 3.7litre)
    2. The conditions and expectations are different.

    Most of the cars driving high mileage on UK motorways are 2litre diesels, fairly recent. The extra fuel use from 70-80 mph is much less than quoted.

    Any sensible discussion of this has to look at road safety, environmental concerns, and police/ council desires to raise money as separate issues and look at them properly. Not make up confused arguments using assertions about one to justify an argument based on the other.

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  7. Brilliant. Thank you all for your informative comments. JoeB, especially, thank you for pointing out that the data was probably American. After some hard looking I was able to confirm that the web site linked to is an American consumer website. This means that the extra expense in fuel consumption due to driving at 80 mph needs to be reduced by 1/8.

    I will put more details in the blog post itself as an edit, later on.

    I totally agree that environmental and road safety arguments should have precedence over the economic argument, but Hammond himself is trying to justify raising the speed limit by reference to these economic arguments. This blog post was an attempt to show how empty that argument is.

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