To my mind there are two kinds of road runs.
I suspect that posters here are talking about those with multiple engines. To my mind the problem with these is that unless the engines have a communal break, you will be very restricted in what you can photograph / video as you will not be able to get ahead of the convoy safely.
Instead, try making contact with your local engine owners. It's not difficult, they will be regular attenders at a rally near you and often their rough locations are given in the programme and I've never been rebuffed.
I've had some marvellous days out and with care it's rather like having a private charter all to yourself on a preserved railway.
You can get a feel for what's possible if you look at some of the videos here:
https://www.youtube.com/playlist?lis...mb9LDzN5nRVbGY
It should be clear which are the 'private' ones.
As you can imagine, they are generally too busy to get meaningful pictures / video themselves and are delighted with anything you can pass their way. A true win - win situation.