Greg is spot on in his reply, Aidan.
The public road used to go along, exactly as you remember. The circuit on race days used to be closed off by big green wooden gates (a wall almost), just above the La Source hairpin. It then went down the hill of the old pit straight (pits to your right) and the old grandstand opposite the pits, to your left. From memory only, you could then turn left at Eau Rouge at the bottom of the hill and / or climb up again, along the Kemmel Straight.
I certainly remember doing just that in the late 70’s and through the 80’s.
Traditional ‘old fashioned’ road based circuits are now few and far between, which is a pity. It’s understandable though, as they were very fast but, ultimately, too dangerous. Spa, at least retains the essence of what it was. The track (and the public viewing areas) has been made safer - a good thing - but it still has the natural climbs and falls which makes it more interesting and challenging for drivers and the paying public alike. Long may it continue, which (judging by the works done) may well be a long time yet.
It’s a big earner for the local community, which is otherwise very rural. The track is used regularly for private events (there was a track day on the Monday) and for several ‘official’ race days, culminating in the Spa Classic at the end of September (which is very good and excellent value) before the season closes with I think, the 2CV 24-hour race, which I’d like to see before I shuffle off.
Besides the racing, what makes Spa ‘great’. I think it’s the friendliness of the staff, from officials right down to the hired in catering staff, the concession holders and the ‘light touch’ security on the entry gates. Everyone was polite and good natured, despite the heat which peaked in the low 40’s C. I can give an example from the ELMS round of last year. You can go to watch, reasonably close up, the cars being checked by the scrutineers. When they have to move a car in or out, the staff are very polite but firm, when they ask people to make space, which everybody does. It’s as if the organisers want the paying public to get as close as possible but still retain a safe distance for everyone to work in. Very ‘old school’.