Right. Presumably a car idling for ten minutes produces less pollution than a car being driven for ten minutes, but a car that is driven for ten minutes and idled for an additional ten produces more pollution than either of them. Any pollution produced by cars idling in bad traffic is superadded to the pollution produced in transit so improving the flow of traffic should reduce pollution even if the total number of cars remains steady.
If the trip costed 10 minutes moving, yes the comparison would be between a car moving for 10 minutes and one that idles for some time and then moves for 10 minutes. But congestion makes the cars move slower, and at congestion speeds the amount of pollution increases very quickly with reduced speeds.
