My question here is, do you mean freeways or not? If so, as far as I'm concerned any road which cannot be actually enjoyed at sub-triple-digit speeds doesn't count. If you don't, then that's actually more of a reason not to ride them there. Sure there is, if you want to be incredibly onerous and annoying to deal with under any and all circumstances. The gulf between the capabilities of a bicycle (or worse, a pedestrian) and the capabilities of a car is so wide that they can never share the same road surface without inflicting obnoxious restrictions on each other - but mostly on drivers. And once again, we see the tragedy of treating efficiency as an end in itself, rather than a means to an end. Efficiency has many definitions to many people in different situations and focusing on the most abstract ones (space and fuel) ends up making everyone's life less efficient in the end.