Once or twice a week, encouraged by my wife, I hop on my bike and pedal a few km down to a local gym. On the way I have to cross South Road, which is a major arterial road here in Adelaide. The only safe way to get across is using the pedestrian crossing at a set of traffic lights.
Something has been bothering me about this pedestrian crossing – how much fuel was I wasting by crossing that road? Today I decided to quantify my fears with some numbers – when I pressed the button I counted about 40 cars and 10 trucks up to semi-trailer size that I had stopped. I estimate that a total mass of 40(1,300) + 10(10,000) = 152,000 kg needs to be braked to a stop, then be accelerated back up to 50 km/hr by burning fossil fuels.
Now 50 km/hr is about 14 m/s, which means the total energy of this moving column of vehicles is 0.5*m*v*v = 14.9MJ. So 14.9MJ is required to take the mass from 0 to 50 km/hr, which must come from fossil fuels. I think petrol has about 32 MJ/litre and I estimate that an internal combustion engine is 10% efficient in converting fossil fuel to kinetic energy at the varying loads required under acceleration. For the sake of argument I will assume all the vehicles run on petrol, although the trucks would of course be diesel and a few of the cars LPG. Anyway this means to take my bike across this pedestrian crossing requires 14.9/(32(0.1)) = 4.6 litres of fossil fuel, plus some fuel consumed in idling and a few extra minutes to the journey of at least 40 people. At current prices that’s about AUD$6 of fuel, or $12 for the two way trip to the gym. Although to be fair today a fellow biker crossed with me so lets call it $9 for the round trip.
My conclusion is that the world supply of fossil fuels declines by 4.6 litres every time I press the button on that pedestrian crossing.
In comparison if I had taken my inefficient 6 cylinder internal combustion car (about 7km/litre on a short trip in traffic) I would have used about 1 litre or AUD$1.20. My car would travel with the traffic and not cause a red light at a pedestrian crossing. If I had taken my electric car then about 28 cents in electricity (at current peak rates here in South Australia) would have been used. I draw no conclusion from this, as intuitively using a bike is much better than even an electric car.
To be honest 14.9MJ and 4.6 litres seems low, can anyone suggest a different way of working this out or spot an error in my estimates?