Thanks for pointing this out, I was trying to figure out why my brain didn't like per mile for things moving at vastly different speeds.
If my job is 2 hours away because there is no motorway, I will get a different job. If there is a motorway, making the commute 40mins, I will take that job.
People care how long the journey is, not how far the journey is.
People live by commute time, not commute distance. Same for service areas, if the roads all have low speed limits you would need more drivers, they wouldn't just drive for longer.
Accidents per hour of driving would be more useful for public policymakers – given fixed average commute 1 hour per day, what's the tradeoffs of building highways (worker mobility, commerce, pollution, landscape, accidents).