I think that depends on the motivations of the driver. You (and I) are probably thinking of a trip that is motivated solely by getting from A to B (or A to B to C to A). In that case, any pollution from idling is strictly additive.
But a taxicab working an 8 or 12 hour shift is about the only case where I think GP's math/logic applies. (And to be fair, there are a damn lot of yellow cabs in Manhattan.)
But a taxicab working an 8 or 12 hour shift is about the only case where I think GP's math/logic applies. (And to be fair, there are a damn lot of yellow cabs in Manhattan.)