It´s the classical market sizing, and there are many ways to tackle it.
PM me if you want, since one of my cases precisely is about the privatization of a highway, including a market sizing on the number of users.
One of many options:
- Total population in London and surroundings
- % of them who transit on a weekly basis
- Working population
- Weekend population
- % of them who do so by using the motorway, splitting
- Private cars and car sharing
- Perhaps some trucks or commercial vehicles on top
Hope it helps!