Sunday, August 1, 2010

Going green through Amber

We’ve just returned from holiday.  Visiting Luxembourg reminded me of an article about Boris Johnson’s efforts to make London more green by changing all the light bulbs in their thousands of traffic lights by changing them for LED bulbs.  While I applaud Boris’ initiative I believe he is missing a trick that Luxembourg and many American cities areadly know i.e. the use of flashing amber during non peak hours.

Why is this greener?  Its simple laws of physics.  Consider a car travelling at 28 miles per hour having to stop at a red light.  28mph = 12.5 meters/second.  For a 1.5 tonne car the energy put into the breaks during stopping is (0.5 * MV2) 118,000 Joules.  Now let us assume 40 cars per hour arrive at the traffic lights and half of them get a red light. This gives a total number of Joules lost per hour of 2.35MJ or 0.65kWh.  Four traffic lights per junction burn perhaps 600W or 0.6kWh so more energy is lost in the cars brakes than the bulbs. 

The story does not end there though.  Imagine its night and the cars headlights, heaters, or worse aircon are on and the engines are idling.  Its probably consuming about 300W of electricity and 200W of mechanical energy keeping the engine running.  If those 20 cars are 30% efficient and each stopped for an average of 30 seconds this contributes another 0.28kWh.  To be realistic we should also assume that every car has to slow down at the junction, loosing around 40,000Joules or another 0.23kWh. The total energy consumed per junction per hour is therefore 0.6+0.65+0.28+0.23 = 1.75kWh per hour.

Now let us consider the flashing amber scenario.  Assume now that only 10% of cars have to give way to other cars. We could be more scientific based on Poisson distribution, the average arrival rate of cars and the time it takes to cross the junction but I think 10% will be illustrative.  This means that 90% of cars slow down but do not stop and 10% of cars stop.  Assuming that the amber lights are now on 35% of the time the new energy consumption is 0.73kWh.  Lets now compare this figure with Boris Johnson’s green initiative, which comes to 1.25kWh assuming LED lights are 6x the efficiency. Taken together we can get down to 0.57kWh per junction or a 66% saving. A roundabout would be even more energy efficient as even less cars would stop and no lights would need to be burning.

Of course the traffic control zealots will argue that flashing amber lights are less safe than proper controlled lights.  I believe that this view is an insult to drivers who quite happily negotiate thousands of non traffic light controlled junctions every year without incident.  And yes, there probably will be a few more accidents per year, but compared to the energy savings achieved it will be a small price to pay.  Besides this there is some evidence that removing traffic lights actually reduces road deaths, see this article and another of Boris’ initiatives here for instance.

It is these same zealots that, in 2010, are still installing timer based lighting that give green lights to empty roads when cars are waiting.  If they really believed their rhetoric they would be spending 50W per junction on electronic control systems (for instance see , a 3D camera start-up) that can recognise where the traffic is and make the lights green for as many vehicles as possible. Such systems could automatically measure traffic density and, when appropriate, switch to flashing amber or even (for those notorious traffic light enhanced roundabouts) off. 

I hope I illustrated that holistic outside-the-box thinking with proper weighting of risk and reward will get us much further than simple incremental fixes. Its time to weadle out the zealots and install a new generation of traffic planners.

1 comment:

Stephen Ellwood said...

4 years later its starting to happen: