Yes, my calculation is one-directional. Two directions just doubles both costs and profits (and is probably necessary to make it appealing, and for fire safety reasons).

The bigger problem is that I casually listed average vehicles per minute, but in practice you likely have barley any traffic at night and four times the average during peak hours.

Also "finance over 50 years for free" is a giant stretch. Even at a theoretical 0% interest rate the opportunity cost makes this much more expensive in reality.