^^ I don't think exhaust will be a big problem if diesel buses are used on Carroll. Much of Carroll already sees vehicle exhaust, from loading docks and semi trucks to valets parking cars. Ventilation is only a concern where the space is well-sealed from the outside atmosphere, like in a subway, or when the volume of exhaust is tremendous, like along an expressway where congestion occurs (e.g. Hubbards Cave).
The Silver Line in Boston is a useful analogy, but only to a point. The Silver Line's tunnels are very tight, cut-and-cover subway tunnels woven around the Big Dig, the Red Line, and numerous building foundations. Carroll is really just a small street that got partially decked over, with many open grates and expansion joints for air to circulate though.
From an engineering standpoint, it's a fairly easy project... repave Carroll with a uniform surface, stripe it for bus operations, and build vertical access to the streets above at the stations. The challenging part is getting all the landowners along the route (many of which are major, major players) to accept the loss of their alley. That's why they pressured the city to examine using Lower Wacker instead.
Originally Posted by orulz
BTW, isn't there a plan for a grade separation under the railroad on Clinton Street as a part of this transitway?
Yes. Early plans called for the crossing at Canal to be closed and an underpass to be built at Clinton (probably a low-clearance one, considering the other things that may be built beneath Clinton).
I've now heard that planners have abandoned the idea of a subterranean busway level in the WLTC, making it three levels instead of four: pedestrian concourse, HSR, subway. This would significantly reduce the cost of the project.
Buses would instead run on the surface... I'm not sure if "on the surface" means a bus mall, or simply dedicated lanes. I've always thought a Clinton bus mall might work nicely. The State Street mall was a failure, but only because of poor design, and because of other forces conspiring to push the Loop into decay... I don't think it disproves the entire concept. Of course, you would need to work with the business owners along the street... like allowing after-hours access to trucks for deliveries so that Clinton can remain a busy retail street.