Stopping Traffic

In the past I’ve written about the fuel consumption of a person (about 300 mpg), and also why measuring fuel consumption in miles per gallon is the total inverse about what is sensible (it’s more a measure of fuel efficiency, not consumption).

Today I want to look into how much energy is wasted when cars have to, unnecessarily, stop.

We’ve all been there; you stand at a pedestrian crossing, push the button to cross, and patiently wait for the lights to change. Nothing happens.

Now, I’m not suggesting you are a jaywalker, but if you look both directions, and nothing is coming, then look again, how many times have you just crossed the road anyway?

The problem is that there is no way to cancel your request for the lights. Have you ever reached the other side to find the lights cycle as soon as you safely got there, then cars appear? You sheepishly skulk away, avoiding eye contact with the drivers (theoretically, if the lights would have changed the instant you pressed the button, it’s no different, but somehow it feels different because, if you had crossed without even attempting to press the button, this situation could have been avoided). How much energy has been wasted?

Stop lights are terrific ways to turn gasoline into heat and brake dust. When you are cruising in your car at highway speeds, the majority of your usable gasoline energy (that not just being lost as heat) is being used to punch you through the air and overcome air resistance (drag), with other forfeitures to overcome road resistance, gearing losses, and friction. When you slam on the brakes, however, your kinetic energy is quickly transferred into heat (assuming you are driving a car with an internal combustion engine. If you are driving one of those fancy electric or hybrid vehicles, your regenerative breaking may be scavenging as much of this energy as possible to convert back into electricity for your battery).

How much energy are we talking about? As a first approximation, let’s say a car weighs a metric tonne, and is travelling at 40 mph (around 18 m/s). Using the formula for kinetic energy of ½mv² this equates to approximately 162 KJ of energy. This energy is lost, and through reciprocity, as a first order approximation, it will take this same amount of energy to get the car back up to speed again. There are 131,760 KJ of energy in a gallon of gasoline. Theoretically, an internal combustion engine might be able to convert 50% of this energy into forward motion, but practically, the actual efficiency is nearer 25% which means that it will take approximately 0.00492 of a gallon of fuel to get back up to speed (about 1/200th of a gallon or 19 cc). If the price of a gallon is $4.00 this equates to around $0.02 of waste. If you stopped four or five cars for your sojourn, this is about a dime.

Have you ever mashed a crosswalk button and been convinced it’s not actually doing anything to expedite the cycling of the lights to facilitate your crossing? Actually you are probably right! It’s been estimated that only about 10% of these buttons are actually connected (and even then, only during less busy time windows).

Why? Well, modern traffic control management systems are pretty sophisticated organisms, monitoring thousands of parameters to allow smooth tidal flow of traffic over large, tightly connected, regions. Signal changes needed to be coordinated and synchronized to stop bottlenecks, pressure waves of traffic, and other unexpected side-effect consequences downstream of actions of your crossing request. An unexpected pause, on demand, at one location might be a proverbial butterfly wing flap that has catastrophic chaotic consequences elsewhere. Pushing the button might placate you, illuminate the ‘wait’ lights, and trigger the audio cues to assist disabled people navigating, but will probably not accelerate the time it takes the lights to change.

Cross walk buttons are not the only perpetrators of this deception. Elevator ‘door close’ buttons also have placebo functionality (unless specifically enabled by the building or firefighter overrides), and the doors will not close any faster if you push the button than just wait.

It’s not unheard of for thermostats in certain locations to have similarly Svengali functionality; giving you the illusion of control, when in reality they are buffering, throttling, or overriding your demands in the interests of a bigger picture.

Of course, as the number of self-driving cars increases, there is a a good chance that this technology will turn pedestrians into jerks.