I can't quote a source but have seen such at one time. Bridges ARE required to open according to their rules on time of day,season,etc, upon sounding of proper signal or communication,etc. Likely the CFR has such written rules somewhere for navigable waterways...towns can't block waterways willy nilly. I'd guess even the bridge clearances when open and closed must meet federal requirements. The USCG determines which which bodies of water fall under federal jurisdiction. Page 102103 of Chapman's discusses bridge protocols. A bridge operator can refuse to open if you are required to lower a movable antenna, for example, but they can't dispute your measure of your required fixed clearance...anymore than you can dispute their bridge clearance. If you are given clearance to pass and hit the bridge because the clearance is less than promised, the operator is liable. The amount of 'safety clearance" required is debatable, but as noted often varies over the width of the span and with the type of bridge. Waves, fuel load,current, boat speed and even wind can all affect boat height and the ability to safely pass...those and safety clearance are all the captain's responsibility. The other side of the coin is that the operator knows there is some increased danger to vehicle or train traffic with the bridge open, so they are quite right to use caution and require us to follow the rules as well. Rob Brueckner Hatteras YF