Pulling a Fast One
Will motorists fall for the Department for Transport’s latest plans to increase the motorway speed limit and flagging sales of petrol and diesel?
As September segued into June and another UK temperature record fell the Transport Secretary Philip Hammond announced plans to consult on raising the motorway speed limit from 70 to 80mph.
The announcement coincides with government figures showing a slump in sales of petrol and diesel which according to the AA has deprived the Treasury of almost £1bn in fuel duty over the first six months of the year (1).
Increasing the limit by 10mph means that given an empty stretch of motorway, you could shave very nearly eleven minutes off a 100 mile journey (2). The Government thinks this will bring ‘huge economic benefits’ and ‘hundreds of millions’ of pounds to the economy. But don’t start planning how you’ll use all that free time just yet.
Contrary to what might be expected a higher speed limit is more likely to increase congestion and journey times than reduce them.
As speed increases so too do the typical (safe) stopping distances between vehicles (3). This reduces the number of cars you can slot into any stretch of road. Move up from 70mph to 80mph and it drops from 16 cars per mile (4) (of motorway lane) to 13 (and just 6.5 when wet and stopping distances double). That may not sound much but that’s 300 fewer cars per 100 miles of motorway lane on a dry day which adds about another mile (5) of tailback leading to more not less congestion.
Then there is safety. We can expect a big dust-up between the petrol heads and road safety campaigners on this one. But whatever the arguments about improvements in vehicle design, there’s some basic physics which may be a bit dry, but can’t be ignored.
The energy of a moving vehicle increases in proportion to the square of its speed. This means that a one tonne car travelling at 80mph has 35% more kinetic energy than one travelling at 70mph (6). So what? In energy terms that’s equivalent to dumping 350kg (7) of cement in back of the car and driving at 70.
That’s a lot of extra energy to get rid of in the event of a crash.
Given that people’s reaction times don’t speed-up as you go faster, it will be interesting to hear the views of the NHS and emergency services on a higher speed limit as they will be ones picking up the pieces, and the cost of any reduction in safety. The Department for Transport certainly used to think that speed was a significant risk factor in road safety. Why else install speed cameras and run TV adverts reminding us that ‘Speed Kills’?
So what’s behind this proposal? The real reason may be to do with fuel consumption, and another dry but revealing characteristic of moving vehicles.
The power required by a car on a motorway is roughly proportional to the cube of its speed. What this means in practice is that at 70mph a typical car requires about 80kW of power to push it along. At 80mph this increases one and half times to 120kW (8) of power. (That’s the same power requirement as 120 one bar electric fires turned on at the same time, or 1200 incandescent 100W light bulbs). More power means more energy consumption per hour (see footnote (9) for just how much) and more fuel consumption.
So raise the speed limit for the fleet of cars and motorcycles in England and Wales and at a stroke you dramatically increase the amount of fuel consumed. Of course officials in the Department for Transport (at least in Scotland), know this. There you’ll find signs on the motorway encouraging drivers to slow down to 60mph to save fuel which indeed it does by more than halving the power required to drive at 80mph (10).
So it’s difficult to escape the conclusion that this is simply a ploy to put more money in the Treasury’s coffers by getting us to buy more fuel.
Aside from the fact that it is likely to achieve very little of positive benefit, it drives a Chelsea Tractor through the Government’s claim to want to be the greenest ever and to be serious about responding to climate change. Will the emission increases from higher fuel consumption be met by reductions elsewhere in the economy? If so from where? And does this indicate a shift in policy so that action on climate change is no-longer a priority and government departments such as the DfT may now treat this as optional?
Climate change is as real as the economic crisis. It requires real solutions that add up, and which cannot be put on hold until the economy is fixed. It is possible to align the solutions to both the financial and climate crisis, thereby cutting emissions, improving our energy security, saving money and creating new jobs. Raising the speed limit does none of these. Insulating the nation’s homes so they are fit for purpose, rebuilding our energy infrastructure to name but two, could.
At a time when the pain of government cuts is being felt across the country it is galling to see time, effort and money being diverted into a flawed consultation.
© Climate Works Ltd, October 2011.
Footnotes and sources
(2) Time = distance/speed
So at 70mph time = 100/70 = 1.43 hours = 85.8mins
And at 80mph time = 100/80 = 1.25 hours = 75mins
Difference = 10.8mins
(3) Safe stopping distance at 60mph (dry) = 73m
And at 70mph (dry) = 96m
Source: Direct gov – The Highway Code. http://www.direct.gov.uk/en/TravelAndTransport/Highwaycode/DG_070304
Estimated stopping distance at 80mph = 400ft = 122m
Source: Various sources. Consensus seems to be that typical (dry) stopping distance at 80mph is 400ft.
According to the highway code stopping distances are double in wet conditions.
(4) Cars per mile.
Assume length of average car is 4m
At 70mph car plus typical stopping distance is 96m + 4m = 100m
Cars per mile = 1609/100 = 16
At 70mph (wet) car plus typical stopping distance is 192 + 4 = 196m
Cars per mile = 1609/196=8
At 80mph car plus typical stopping distance (dry) is 122m + 4m = 126m
Cars per mile = 1609/126=12.8=13
At 80mph (wet) car plus typical stopping distance is 244m + 4m = 248m
Cars per mile = 1609/248 = 6.5
1 mile = 1609m
Source: Average length of car – Highway Code.
(5) Additional tail back with 80mph speed limit.
Assume average car is 4m in length and average space between cars is 2m.
300 cars require 300 x 6m = 1800m = 1.1miles
(6) Kinetic energy of a moving vehicle = ½mv⌃2
So for a one tonne car moving at 70mph (31 m/s) the kinetic energy of the vehicle is:
1/2 x 1000 x 31 x 31 = 480,500 Joules = 0.13kWh
And at 80mph (36m/s) the kinetic energy is:
1/2 x 1000 x 36 x 36 = 648,000 Joules = 0.18kWh
((648,000-480,500)/480,500) x 100 = 34.86% ≈ 35%
(7) To increase the kinetic energy of a one tonne car travelling at 70mph (31m/s) add 350kg to the mass of the car.
New mass of the car = (648000/(31 x 31)) x 2 = 1349kg ≈ 1350kg
(8) The power consumed by the engine is estimated to be roughly:
4 x ½ρAv⌃3 where A is the frontal area of the car, v is speed in meters per second and ρ is the density of air (kg per cubic meter).
Assume the frontal area of a typical car to be 1 sq meter and the density of air to be 1.3kg per cubic meter
So at 70mph (31m/s)the sum is:
2 x 1.3 x 1 x 31 x 31 x 31 = 77.5 kW ≈ 80kW
And at 80mph (36m/s) the sum is:
2 x 1.3 x 1 x 36 x 36 x 36 = 121kW ≈ 120kW
And at 60mph (27m/s) the sum is:
2 x 1.3 x 1 x 27 x 27 x 27 = 51kW ≈ 50 kW
Source (equation): Sustainable Energy Without the Hot Air, Cars II, page 256, MacKay, David, J.C.
These calculations ignore rolling resistance which increases the power requirement by about 15% when travelling at constant speed.
(9) How much energy is required to drive 100 miles at 60mph, 70mph and 80mph?
At 60mph it takes 1.67 hours to complete the journey.
At 70mph 1.43 hours and at 80mph 1.25 hours
The energy needed to make the journey is given by the equation: Power x time
So at 60mph the energy = 50kW x 1.67 = 83.5kWh
And at 70mph = 80kW x 1.43 = 114kWh
And at 80mph = 120kW x 1.25 = 150kWh
In each case we ignore rolling resistance which adds about 15% to these figures.
So driving at 80 rather than 70mph adds roughly 36kWh to the energy required to push the air out of the way. But these figures ignore the efficiency of the engine.
If we assume this is 25% then the energy required to push the car along (again ignoring rolling resistance) is:
At 60mph: Energy required = 83.5kWh x (100/25) = 334kWh
At 70mph Energy required = 114kWh x (100/25) = 456kWh
At 80mph Energy required = 150kWh x (100/25) = 600kWh
A one bar (1kW) electric fire will consume 1kWh of energy if running for one hour. So the energy required to complete 100 miles at 80mph is equivalent to running a one bar fire for 600 hours or 600 one bar fires for one hour.
(10) – See footnotes 8 and 9 above.