QUOTE=tcs;20705709]Tunneling is expensive, even with modern TBMs.
Wondering: what's the smallest dimension bike/pedestrian tunnel you'd feel comfortable riding through?
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I can tell you that a 4 meter wide and 2+ meter high is plenty of space. They are currently reconstructing the Maastunnel in Rotterdam with it's bicycle tunnel;
Maastunnel Rotterdam - BicycleDutch
