Recently I was driving two rental vehicles in Europe, both with standard transmissions. One was a Volvo V60 in Scotland, and the other a VW Passat in Germany. They all had a feature that will be unknown to most North American drivers because the vast majority of vehicles in North America has automatic transmissions. Due to much higher fuel prices, drivers in Europe prefer manual transmissions because of their better fuel efficiency. To make these vehicles even more fuel efficient, many manufacturers equip their cars with a stop-start system that switches off the engine when the car is stopped. It works quite easily. When a vehicle is stopped at a red light, the driver depresses the clutch and the car is taken out of gear into neutral, the engine switches off. The dashboard display shows the symbol above: the letter A with a circular arrow. When the clutch is depressed again, the engine restarts. The engine may also start up on its own if there is power demand from other systems, such as the air conditioning system. To make this work well, vehicles with stop-start systems are equipped with stronger batteries. Toyota pioneered this technology in the 1960s, but it was Volkswagen which introduced this feature into its vehicles on a larger scale in the mid-1990s. As a driver, it doesn't take long to get used to the feature. One may also enjoy a few moments of quiet when the car is stopped at a red light. But do such systems benefit the environment and save fuel? Under which conditions are such gains larger or smaller?

To gauge the benefits of stop-start systems, especially in urban environments, we need to know: (1) the average time spent stopping at traffic lights, as a percentage of a trip's duration; (2) the fuel consumption in litres per hour while stopped; (3) the total annual duration in hours spent on urban trips. The United States EPA has a test procedure known as FTP-75 for a city driving cycle, which simulates driving a distance of 17.8km ovr 31 minutes at an average speed of 34.2 km/h. In this test cycle, a car spends 12.86% standing (stopped). in a similar European test procedure, the New European Driving Cycle (NEDC), a vehicle spends 20.42% standing. A typical car consumes about 0.9 litres of gasoline per hour idling. Further assume that a typical commuter spends 30 minutes in the morning and afternoon driving to work and back home, 250 days per year. Multiplying these three numbers suggests that that driver spends about 32–51 hours stopped in traffic per year. This consumes about 29–46 litres of gasoline per year. This may not sound like much, but if many vehicles employ stop-start technology, reduced fuel consumption could help reduce air pollution in urban areas. Realistically, use of stop-start systems will not be as perfect as in test cycles. Practical road tests conducted by automobile associations and other consumer organizations suggests that savings in the 5-10% range can be expected.

The largest benefits of stop-start systems in automobiles accrue in urban areas where frequent stops are common. Some of the pollution is particularly concentrated around busy intersections. Stop-start technology can help reduce such urban pollution hot spots. The higher cost of stop-start systems (about $250-300) amortizes over a few years. So when you buy your next manual-transmission car, consider one with stop-start technology. Stop idling, save fuel, and reduce emissions. There is just one caveat: few automakers have embraced stop-start technology for vehicles with automatic transmissions. However, this technology has arrived in North America through another channel already: hybrid-electric cars all have stop-start systems. If you prefer automatic transmissions, a hybrid-electric vehicle is your best option to benefit from stop-start technology in North America.