The tunnel costs are predictable because the tunnel is all in rock. Using the experience of tunneling in the Alps, the cost of the tunnel is predictable.
The cost of the tunnel for Bus Rapid transit is considerablely less for BRT because the stations are shorter, the tunnel is not as deep and not as long. As well, the tunnel does not need to be as high as LRT because LRT requires overhead wires in the ceiling.
It's the portion of the proposed LRT plan between Tunney's Pasture and Blair Road that will skyrocket and will result, even if we use the present estimated costs will result (even with the gas tax) in a requirement to borrow 1 Billion dollars.
BRT which only requires the tunnel is at no cost to the taxpayer.