You can design a bus for these tunnels fairly easily. However, 140mph under a city vs 3 mph at street level and buses lose unless they have 47+ passengers but a 50 passenger bus costs 2-3 car slots easily so it's a wash.

PS: I-66 really does have 3mph traffic as it will take 30+ minutes to travel 10 miles. As in I was happy if that part was under 30 minutes, and not surprised if it took 40 minutes as the average was well over 30.