The reintroduction of a ring-fenced road fund seems very strange, unless it is to protect the roads budget from other cuts.
If it were intended as a headline-grabber to position the Tories 'on the side of the motorist', then that too seems odd as this sort of presentational trickery is usually reserved for pre-election periods. Any small boost it gives them in (some) public opinion will be long forgotten by 2020.
You might find
this article by the Campaign for Better Transport interesting. Here's a snippet:
Read the small print - and with Budgets, the small print is what matters - and a different picture emerges. For a start, it's quite clear that the "Road Fund" will (in England anyway) only go to strategic roads - in other words, the 3% or so of English roads run by Highways England. The rest of the road network will have to rely as now on ordinary Government funding.
This is part of a wider policy. In the last Parliament, there was a lot of work done on part privatising the motorways - but to make this work it was realised that a secure revenue stream would be needed. Lots of work was done on putting tolls on new roads, starting with the A14 Cambridge-Huntingdon upgrade, but the politics of that proved too toxic. With this new Road Fund, the job is done - and Highways England can be privatised, in whole or part, with a revenue stream against which the new private utility can borrow to build more
GC