The Silver Line in Boston is exactly that: dual mode buses (http://en.wikipedia.org/wiki/Dual-mode_bus http://en.wikipedia.org/wiki/Dual-mode_bus), and part of

The Silver Line in Boston is exactly that: dual mode buses (http://en.wikipedia.org/wiki/Dual-mode_bus http://en.wikipedia.org/wiki/Dual-mode_bus), and part of the line is a dedicated tunnel for the buses, while the other part is normal streets.