The heath was a place where people walked their dogs (real or imaginary). Quite when the motorway arrived is speculation.
It's rather a bleak, flat place so I suspect non-dog owners didn't visit it much, except to dump illegal waste or for the kind of illicit tryst that other heaths - such as Hampstead Heath - are famous for.
I took my dog Bumpy on a heath once and he ran off. I stomped around the place shouting 'Bumpy, Bumpy', my calls luring out several solitary men from the surrounding copses, but no dogs.
It was suggested within my family once that my father was gay, and that his failure to 'come out' was what destroyed his marriage and drove him to drink.