That's actually the standard model for evaluating transport projects: aggregating small time savings across millions of people.

You basically take those millions of saved hours and multiply them by a government-standard 'value of time' (roughly £15/hr in the UK). That usually makes up the bulk of the benefits, though they also price in things like safety (a prevented death is worth ~£2m), carbon, noise, etc.

IIRC, if you hit a Benefit-Cost Ratio of 2.0 or higher, the project is considered 'high value' and has a good shot at getting executed.