The Council on Environmental Quality (CEQ) - part of the Executive Office of the President of the United States - has issued draft guidance on consideration of the effects of climate change and greenhouse gas emissions in projects. An assessment is likely to be necessary if a proposed action is likely to cause emissions of 25,000 metric tonnes of CO2 or more.

The draft guidance is on the web-site of the White House (see the CEQ website).

So just in the interests of curiosity, I tried to work out the CO2 impact of Heathrow Express. I was helped by work some colleagues have been doing on the carbon impact of transport generally.

This gave me baseline statistics for fuel consumption (cars - 7 litres/100 km), emissions (petrol car 2.76 kg CO2/litre and diesel 3.17) and the conversion factor for kg of CO2/vehicle-km (0.19 petrol, 0.22 diesel). The emissions for electric trains are 0.05 kg CO2/passenger-kilometre.

The road distance from Paddington to Heathrow Terminal 5 is 29.8km: by rail it's 26.2.

OK, not all Heathrow Express users start from Paddington or go to Terminal 5, but it's a reasonable journey for the comparison.

Multiplying kg of CO2/km by distance gave the CO2 emissions/trip.
Heathrow Express carries 4.93m passengers a year: the emissions related to that amount to 6,461 tonnes of CO2.

Suppose Heathrow Express didn't exist? I assumed that those 4.93m people would access the airport by private car or taxi in the same proportions as other passengers do now. I also assumed the cars were petrol and the taxis diesel powered. Car mode share is 35% and cab 27%, so I allocated 56.5% of Heathrow Express trips to car and 43.5% to taxi.

I assumed that if people accessed the airport by car, the car would come back again - they'd be kiss and ride trips. If they came by taxi, the same cab would be used by different people in different directions. So I doubled the figure for car users.

Tonnes of CO2 emitted by the vehicles of road users hypothetically unable to use Heathrow Express came to 46,255 - about seven times the tonnage of Heathrow Express.

Wow!

Now ok, there are several heroic assumptions in all this - but it does give an order of magnitude figure.

Moreover, coming back to the original cause of this research, it shows that an airport express serving an airport under 30 km from the city carrying under 5 million passengers a year can save 40,000 tonnes of CO2 - significantly more than the CEQ guideline.