Actually, bicycle tunnels - with a limited slope of course, like all other bicycle infrastructure - are recommended over bicycle bridges. These are more comfortable for cyclists. Gravity helps one cycle down, and the kinetic energy gained helps cyclists on the way up in the second half of the tunnel.
Also they can be smaller and cheaper to build. Because they only carry bicycles and pedestrians, a height clearance of about 2.5 meters is enough, while a bridge needs around 5 meters to let cargo trucks pass underneath. This also results in much shorter ramps.
