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Monday, August 15, 2011

Many people try to be greener by walking instead of riding in a bus or car, and that's great. For pedestrians, crosswalks are a good demand-based way to stop traffic and get safely across the road. However, if you don't use crosswalks judiciously, you're not being as green as you think.

At a local crosswalk where I cross, typically from 10-30 cars will stop to let me across. The speed limit there is 50 km/h, or 14 m/s (although they're usually going a little faster). The vehicle traffic is varied, but in my region of the world there are a lot of pickup trucks. On the low end of mass, a Toyota Tercel car is 914kg, and on the high end a Ford F-350 truck is 4490kg. The approximate average mass of a local vehicle is 2702 kg.

A good measure of the energy "wasted" is the amount of energy required to return vehicles to their previous speed after they stopped for the pedestrian.

E = 0.5*m*v2 = 0.5 * 2702 kg * (14 m/s)^2 = 260 kJ for the average vehicle, or between 2.6MJ and 7.9 MJ for 10-30 average vehicles.

These values might not mean much to the layperson, so I'll put it in terms of gasoline usage.

Gasoline has an energy density of approximately 35 MJ/litre. This means that, given 100% efficiency of transmutation from gasoline to kinetic energy, the average vehicle uses 7.4 ml of gasoline to go from zero to 50 km/h. En masse, 10-30 average vehicles will use a total of 74 ml to 220 ml of gasoline to get up to speed after I stop them.

But it's even worse than that. Most steel internal combustion gasoline vehicle engines have an average energy conversion efficiency of only 18-20% (and a limit of 37% efficiency). That is, on average, the cars and trucks are only using 20% of the energy contained in the gasoline; the rest of the energy is mostly spent on waste heat.

For my example crosswalk, it costs the environment 370 ml to 1.1 litres of burnt gasoline every time I stop traffic to cross the road!

Of course, a product of burning gasoline is CO2 released in the atmosphere, created at a rate of approximately 2.3 kg CO2 per litre of gasoline, resulting in 0.85 kg to 2.5 kg of CO2 dumped into the atmosphere so I can cross the street.

Problem Workaround: Pedestrians have a reasonable need to cross the street. The greenest way to do this at a crosswalk is to wait for a break in traffic so the least amount of cars have to stop.

Don't fall into the entitlement habit of forcing the cars to stop for you at the moment you want to cross; You are legally within your rights to do so, but it comes at a cost the the environment. If you can wait for a minute for a break in traffic, you might have the chance to save an entire litre of gasoline! Remember, Reduce is the biggest, most important aspect of "Reduce, Reuse, Recycle".

Burton MacKenZie



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