This thread was a really good idea!
It would be helpful to post alternatives where possible. In the case of the Conowingo Dam, for example: there's a bridge at Holtwood, PA, a dozen miles upstream.
Conowingo Dam is already a 20 mile detour from US 40. If you are heading more north/south the Holtwood, PA crossing is a good option otherwise the only other option is to get transported across the Hatem bridge (US 40) by
Biller's Bikes