What is the accident and injury and fatality rate in the middle of cities relative to the kinds of collisions that might result between the straddling bus and other vehicles?

Warning systems with overheight sensors and flashing lights can be built into the bus. These might include a fringe of chains or other objects that make noise when brushing the roof of an overheight vehicle say ten feet before reaching the bus, then a girder or lintel protects the bus passenger compartment. As part of the warning, the fringe might drop down well below 6 feet of ground clearance (in lieu of a gate) if the sensors detected an overheight vehicle approaching.

In the accident mentioned in the link, the driver was speeding, going more than five times a speed considered "fast" in the inner city.

One article mentioned that the width of some models would be slightly under 20 feet. The inside width would then be about 17 feet. This is not really enough for two lanes of traffic underneath as cars will encroach on the "center line" (which may be projected onto the pavement from the bus undercarriage) before proceeding under the bus. To work properly, the side carriages of the bus must leave the entire two lane width of original roadway under the bus clear, which in turn requires "bike lanes" on both sides of the bus. Of course we would expect the bus driver to not mow down bikes or lane changing vehicles in plain view ahead of the bus.

Other goodies could be traffic signals under the bus to relay the indications of traffic signals up ahead for cars emerging in front. The sides of the bus are continuous; if the bus happens to be stopped in an intersection, cross traffic is blocked and cars under the bus remain in place.