Traffic congestion is increasing in many cities, leading to longer travel times and more particle emissions from vehicles. Most of these are black carbon but a large fraction also consists of volatile organic compounds. These particles, which can be just nanometres in size, are particularly worrying because they are thought to lead to ailments such as heart and respiratory disease by penetrating deep into the lungs.

Now, using a "line source" method, Lidia Morawska and colleagues have found that the number of particles emitted when a vehicle is forced to stop and start at a red light is much higher than when it travels past the light without stopping.

The researchers came to their conclusion by experimentally determining the particle number emission factors under different driving conditions, such as when cars are cruising, idle, accelerating and stationary (figure 1). They then used these results to model the number of particles emitted from a car for each 10-metre length of road when it cruised past without stopping (at a green light, for example), and when it was forced to stop and start at a pedestrian crossing.

The model revealed that the number of particles emitted increased by more than an order of magnitude when the car stopped and then accelerated away from the crossing. However, it also showed that the number of emissions actually decreased when the vehicle was slowing down to stop at the crossing, compared to the number had it just cruised past without stopping at all.

Next, Morawska and co-workers considered twelve cars per minute travelling in each direction along the road (figure 2). This problem was more complex because the cars do not arrive at the same time and spend different amounts of time idling at the crossing, explains team member Rohan Jayaratne. But, once again, the model showed an order of magnitude increase in particle-number emissions each time a traffic light turned red.

The team repeated its technique for a diesel-powered passenger bus and found a corresponding increase – this time of more than two orders of magnitude. "We estimated that the presence of a person crossing the road increased the number of particles emitted by a car by a factor of four compared with the situation when the car did not stop," Jayaratne told environmentalresearchweb. "A bus stopping at a red light increased this factor to 11."

So how could we reduce such emissions? One obvious way would be to increase the time between red light changes, say the researchers, or decrease the amount of time a light remains red. Another way would be to eliminate the pedestrian crossing altogether. However, these methods may not be generally acceptable, or even feasible, they admit.

More realistic alternatives might include providing underpasses and bridges in place of pedestrian crossings, they suggest. And there might be many ways to reduce emissions from vehicles, such as incorporating after-treatment devices in their design or using cleaner fuel. Driving without accelerating too sharply is also known to reduce particle emissions.

The team now plans to extend its study by looking at other scenarios where traffic is forced to slow down or stop, such as at roundabouts and intersections.

The work was reported in Transportation Research Part D: Transport and Environment.