Well, I was formulating plans for my drawbridges and doors, and came to this conclusion:
Basically, you have a double-wide row of blocks as the frame for the drawbridge, four blocks high. Bottom to top goes a base block, wire above that, a sticky piston, and then a cover-up block which doubles as a fence. Same applies for the second row, which has a base block, empty space, the block that will be pushed out to form the bridge, and a cover-up block. create two of these facing each other with a two block gap in-between, and connect both wires to a common lever. Instant bridge.
Though what I think you're going for, Taxi, is a traditional drawbridge. The only way I can think of accomplishing this is to have an X amount of pistons for X amount of blocks the bridge will be. So if you had a five-block long bridge, you would have a row of five blocks and five pistons behind it. i.e.:
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