Fuel Consumption of a Pedestrian Crossing

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?

12 thoughts on “Fuel Consumption of a Pedestrian Crossing”

1. Fifteen mega-joules is an incredible figure.

I love the analysis!

However, my feeling is that the economic cost of the crossing, as opposed to the energy cost, would be dominated by the time consumed, rather than the (current) price of the energy:

You have 50 vehicles with, say, an average of 1.5 people in each. A 45 second delay for each person would total to about an hour of lost time, or say AUD\$30 in lost time versus the \$6 in lost fuel.

Of course, man-hours are not a limited resource, so it is probably forgivable to spend them a little more freely…

2. The real question is weather this information lets you get out of going to the gym

3. The numbers look a bit big, but then again, 40 cars and 10 trucks is a lot. If there’s this much traffic on that road, then maybe the best solution would be to build a small overpass for bikes/pedestrians.

4. david says:

Leon, you know the people-time is a good point, there might also be a taxi or two that have their meters ticking.

Joel, unfortunately I need to expend much more that 14.9MJ before I can get out of the gym

5. Is this purely a pedestrian crossing, or a road intersection including a pedestrian facility? It seems odd to have a pure pedestrian crossing on a road that busy. A footbridge or tunnel is the usual fayre these days. If its a road intersection, your impact is probably just a drop in the ocean.

I wonder how the pollution levels near intersections compare with those near the smooth flowing sections of the same road?

Steve

6. david says:

Steve – it’s a road crossing that incorporates a pedestrian crossing. However most of the times when I cross it’s just me triggering the lights. Although there is a pedestrian-only crossing about 200m down the road as well.

Adelaide is not well set up for cars (very few freeways for a city of 1.1M people). Which in a way is kinda cool, actually.

A friend of mine from the local Peak Oil group worked out that the energy consumed is equivalent to 23 days of human manual labor!

7. 2 chances to stop the traffic in 200m? Is the city council full of people with investments in the oil industry?

8. peter sun says:

after I have learnt Multivariate regression, I learnt that if you include more variables into your model. you will have differnt conclusion. which mearns, if you add other variables such as: total efficientcy of the society . maybe we will draw another conclusion.

9. David says:

It is a similar problem for railway crossings. Locally (UK) there is a railway crossing that will routinely have 200 vehicles stopped, waiting for the train to pass. That can sometimes be 15 or 20 minutes, if there is a series of trains. The wasted fuel and time is colossal. So a sort of extrapolation is that any crossing of transport systems that interferes with the smooth running of either of them will wate fuel…

10. Bruce Armstrong says:

And if all those vehicles were electric…
They would have recovered between 80 and 90 % of the braking energy so it wouldn’t be lost as heat and brake shoe dust.

I Like the analysis, although it may be a bit worse than that if the standing fuel consumption is also taken into account, say 30 seconds per vehicle. Idling, trucks would be better than petrol cars.

11. pravesh says:

I agree with Jean-Marc Valin, 40 cars are a lot.

Hi David

Now if they had to replace the standard speed humps with pressure plates fixed to a mechanical to electrical charging the pedestrian traffic light system or other.

This would assist .

I am sure much in the way of braking energy can be placed to good use.

As in trams with huge flywheels.Imagine how much energy one can replenish by a loaded 18 wheeler going down a steep gradient.

regards