Here on PirateCraft you have the ability to create working retractable bridges of any size, no command signs needed, They are redstone controlled, giving major flexibility for design.
Build the bridge itself, out of slabs/half-blocks or double slabs.
Make sure there is at least one block next to the bridge where you don’t want the bridge to move, so that it knows what direction to move into.
Power one of the blocks underneath the bridge with redstone. Please note: you are powering a block underneath the bridge, not the bridge itself!
Tip: Running redstone wire underneath a block will power it.
If you power more than one block, the bridge will move faster! Two blocks doubles the speed.
Make sure that when the bridge is fully extended it is still in contact with redstone power underneath. It can be the same blocks, or different ones.
When the blocks receive redstone power, the bridge will move to the south or east (in other words, increasing x or z coordinates; depending on which direction is free), when they lose redstone power, it will move to the north or west (in other words, decreasing x or z coordinates).
Bridges cannot float and do not move entities that are on top of them (This might change).