This is where motorcycle behaviour comes into play: in free-flowing traffic a motorcycle uses the same space on the road as a car, just another slot in a line of traffic, but as the density increases, motorcycles start to use less and less space, eventually disappearing altogether between the traffic queues. The study expresses this as a Passenger Car Equivalent space, or PCE. On an open road a motorcycle has the same value as a car, 1, but as the traffic comes to a standstill it drops to 0, where the bikes are filtering through stationary cars and in effect using no road space, or at least none that’s contributing to congestion. It’s a variable that has some major knock-on effects.