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UK planning to trial 80 mph speed limit

30 April 2012

The UK Government is planning to trial 80 mph (129 km/h) speed limits on sections of the motorway network.

The Government announced a consultation on motorway speeds in September. At the time, then-Transport Secretary Philip Hammond said that the current limit of 70 mph was out of date and that the higher limit would give the economy a boost. The proposal was immediately criticized by environmental and safety campaigners who said it would lead to higher carbon emissions and more road accidents.

In a Parliamentary answer to a House of Lord’s Question last Autumn, Lord Shutt of Greetland said on behalf of the Government that CO2 emissions increase by around 14% for a vehicle driving at a steady 80 mph compared with a steady 70 mph.

April 30, 2012 in Brief | Permalink | Comments (7) | TrackBack (0)

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So how does the 70 to 80mph change yield 14% increase in fuel consumption?

@Paul - good question. Given that the dominant factor in energy consumption at freeway speeds is aero drag and aero drag goes up with the square of speed, increasing speed from 70 mph to 80 mph increases the amount of drag on a car by ~50% - much more than the 14% increase in speed might indicate.

Since there are still other factors that affect range - in real life going from 70 mph to 80 mph will typically increase fuel consumption around 20% while getting you to your destination at most 14% faster.

Fuel consumption is highly dependent on vehicle choice. Fuel tax is enough in UK to give incentive to buy smart. Mandating overly sedate highway speeds is a drag on the economy.

@Dave R: For a given road load, the fuel economy depends on the BSFC map. Although power consumption increases with road speed, engine efficiency may increase depending on the specific characteristics of the engine. Since for most vehicles 70-80 mph is not max power, the actual difference in fuel consumption may be minimal. That depends on the BSFC map and transmission.

The most mad proposal for a county spending a fortune for windmills to reduce CO2.

The most effective way to reduce fuel consumption by automobiles is to reduce the speed. The next most effective way is to use smaller engines. If you have a big enough engine to go 80 mph, then the engine is too big.

The best way to give the UK economy a boost is to loan money to build coal to gasoline factories that can also use natural gas when cheaply available. The factories should be built right away and employ thousands of people in construction alone. We also can have lots more people working in the pits again whilst no money leaves the country for imported price speculated automobile fuel.

London already has too foul of air from automobiles. More vehicles rushing to town are not needed. Fast gasoline powered automobiles produce more pollution per mile and the energy efficiency is dismal as is the CO2 release per person mile. Perhaps full electric vehicles could be allowed to run at 80 mph. ..HG..

Lord Shutt of Greetland has never been in a heavy wind and does not know wind resistance or physics of bodies moving in the air or any other fluid. People rushing home at 80 mph to sit in front of the telly is a real drag on the economy. Goods on lorries should be containers on rail anyway. ..HG..

See article on Net about 3000 mpg micro car from CAL POLY with max speed of 25 mph. It is an entry to the eco-marathon. ..HG..

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