Ride behind her on her bike

"We'll take your bike, and I'll ride behind you to cover your ass." You tell her.

"Okay, that seems like a reasonable plan." She agree.

You both climb down from the roof, to where Batgirl stowed her bike. She swings her legs over and sticks her ass out before looking over her shoulder at you, expectantly. You climb on behind her, squeezing right up to her ass. When she starts the engine the vibration causes her ass to rub against your dick and it gets very hard, very quickly. By reading her mind, you know that she is aware of your erection but has chosen not to mention it. She speeds off in the direction of the bank and in no time you pull up to the rear exit you left from.