Bad maths

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Tom Baker’s recent letter advises the use of simple arithmetic but makes a fundamental arithmetic error. He states that driving at 20 mph uses 50 per cent more fuel than driving at 30mph because it takes 50 per cent longer to get there. Wrong!

Fuel consumption (let’s use old units) is measured in miles per gallon. It compares volume to distance, not time. If a gallon gets you 50 miles, it gets you 50 miles whether it takes you a minute or a week.

Mr Baker then reassures us that some vehicles will increase pollution by dropping down a gear at 20mph, but he doesn’t say that many vehicles will not, in which case they will actually reduce consumption.

The overall effect remains unknown because of the complexity of various vehicles and driving styles, but one thing is undisputed, speed humps cause much more pollution than 20mph limits.

Mr Baker asks do we care about pollution. Yes, but the point of 20mph speed limits is to save lives and his letter makes no reference to this.


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