Jim is correct - I measure as if the all the cars will magically disappear on race day. If the city decides to help an event (got to be some payoff, somewhere, I would imagine!), they would cooperate in having the cars moved. So, it is possible the cars will be gone.
Even if half are gone, it depends on where the cars are parked. If they are parked perfectly, there will be ample opportunity for runners to run the proper SPR. So, one must measure for the SPR on the intended route, whether we know for a fact that cars will be removed, or not.