The sensors do not know the difference between wheelchair trucks and standard (which explains the four large straps used to secure wheelchairs.)
The bridge is rarely turned off for efficiency reasons, this is typically only during the height of Christmas week into New Years. Most likely it was out-of-commission for technical reasons.
When the bridge is turned off the driver gets no green or red light signal, however sometimes a green light will still lead to no bridge show. The script instructed drivers to build up "Cross your fingers, everyone!" It collapses and the driver asks "Hey, who didn't cross your fingers?!?". However when the bridge does nothing, it is a bit awkward to say the least. I am fond of, "Wait a minute, that [crossing your fingers] actually worked?!?"