I disagree Duke, the car parks in Marple are the opposite of "scarcely populated" perhaps you've been visiting them in the wee small hours, does the duchess know about this?
A couple of weeks ago (it was Friday about 1pm) I couldn't get a space in the co-op so I parked in Chadwick Street and it was three-quarters full. My point is that when the supermarket arrives the effect will be that traffic that normally disperses from Marple will now converge upon it. I don't know anything about traffic design schemes and I don't think that you do either but Belly seems to be knowledgeable and he appears to be worried. So I'm going to have to side with him in the hope that he's wrong and you're right.