As a bicyclist, it might make sense, but not commonly. In rural or undeveloped areas, it's cheaper to just pave a bike path aboveground. In dense and developed areas, the sheer number of potential destinations means you'd need an unreasonable high number of points from which to emerge from the tunnel; better to just build protected bike lanes where bicyclists can transition to ordinary sidewalk-borne pedestrians by merely stepping off their bike. The only tunnel solution that makes sense would be a "highway" through a dense area with only few onramps/offramps, but that implies that you have a very high amount of urban density such that bike trips (which are usually under five miles) can't just be done using dedicated above-ground protected bike lanes. Or hell, an actual subway.