Question: My drivers-ed instructor told me if I obeyed the speed limit I’d find most of the stoplights are timed so they change to green just before I get there. He also said I’d save a lot of gas if I drove like that. His explanation didn’t make much sense to me. Is that for real? With gas prices going through the roof I’m interested in anything that will save gas. — DE, Kannapolis, NC
Answer: You have two questions. The first is easy. Yes, in urban and suburban settings traffic lights along major thoroughfares are commonly timed so that, provided you drive close to the speed limit, you’ll hit mostly green lights. Some intersections use what’s called “demand actuated” stoplights that detect the presence of vehicles via magnetic or optical sensors. And in some cities, emergency vehicles carry transponders that can trigger stoplights as needed.
City traffic planners use these strategies for many reasons: fuel efficiency, traffic throughput (vehicles per hour), driver safety, emergency response time, and gridlock avoidance. Increasingly, AI is being used to help meet these goals by anticipating things like rush hour and road maintenance, and reacting to road closures from accidents or weather. But generally speaking, you will see more green lights if you obey the speed limit. The animation shows what happens to two cars with different driving habits. The green car is obeying the speed limit.
Your second question requires some physics.The short answer is yes — you will save fuel if you drive like the green car, and waste fuel if you drive like the black car. Moving bodies have a property called kinetic energy. The amount of kinetic energy depends on the body’s mass and speed according to: KE = ½mv2. To do the calculations we’ll start with this list of assumptions:
- The green and black cars are otherwise identical SUVs with a mass of 2177 kg (class average 4799 lb).
- Both cars have modern internal combustion engines burning regular gasoline.
- Both cars operate with the average fuel-to-tire energy efficiency of 21% (average of range 12–30%).
- Note: The effects of friction (internal, rolling, drag) are included in that 21% efficiency.
- The energy content of regular gasoline = 3.15×107 J/L (joules per liter).
- The “experiment” takes place on a straight and level road with no wind.
- The green car’s speed after acceleration is the speed limit: 15.6 m/s (35 mph).
- The black car’s speed after acceleration is 10 mph over the speed limit: 20.1 m/s (45 mph).
Energy needed to go from rest to 15.6 m/s: KE = ½(2177 kg)(15.6 m/s)2 = 2.65×105 J
Dividing by 0.21 efficiency means the actual energy used = 4.76 × 2.65×105 J = 1.26×106 J
That amount of energy required combustion of: (1.26×106 J) / (3.15×107 J/L) = 0.04 L of gasoline.
You could do that 1250 times on an average (50 L) tank of gas.
Energy needed to go from rest to 20.1 m/s: KE = ½(2177 kg)(20.1 m/s)2 = 4.40×105 J
Dividing by 0.21 efficiency means the actual energy used = 4.76 × 4.40×105 J = 2.09×106 J
That amount of energy required combustion of: (2.09×106 J) / (3.15×107 J/L) = 0.07 L of gasoline.
You could do that 750 times on an average (50 L) tank of gas.
Note how the green car, not having to stop at the second light, is able to preserve its kinetic energy and continue rolling along. The black car, on the other hand, will require another 0.07 L of gasoline to get going again (not to mention addition wear & tear on the engine, drive train, and tires). If these are consistent driving habits the difference adds up over time. Especially if you’re a daily commuter, and in proportion to your commuting distance.
The difference in fuel consumption here was (0.04 L) / (0.07 L) = 43% in fuel savings. In reality the fuel savings that can be expected by driving efficiently is closer to 37%. And that’s for drivers who practice what is called hypermiling. This involves a panoply of driving techniques that collectively yield an average 37% in fuel savings. Read more about hypermiling here:
You’ll see the stoplight timing technique explained here is #46 on their checklist of 109 gas-saving tips.
Most of these hypermiling tactics also apply to electric and hybrid vehicles, many of which have the capability of regenerative braking. This technology stops the car by converting its kinetic energy back into electricity and recharges the battery. With conventional brakes, that kinetic energy is turned into heat in the brake pads, tires, and road.
As fuel costs will continue to rise, you might want to go down that hypermiling checklist and see how many of those tactics could apply for your needs. You’ll save gas money for sure, and it’s good for the planet.
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