I don't know of a way to lower the bridges, I can only offer a couple of tips:
- Buy the "nitrous in every car" ability, I think it's available around level 26. This enables you to really speed up car thefts and will allow you to clear the bridges quickly. I managed to clear a bridge with the garbage truck this way. Fire coming out of a fire truck's exhaust is a sight to behold.
- Buy the "police forgiveness" ability. This greatly reduces the difficulty of escaping with any car, allowing you to gain speed before bridges and clear the gaps without interruptions. This ability must "recharge" between uses, however, but apparently you can just reload your save after usage and it'll be available again. Alternatively
You can also use the mayor forgiveness option if you got it as a reward for the zombie mission
Also, keep in mind that not all bridges are raised to the same height (there is at least one gap that you can clear on foot), and bridges with an elevated track above them are significantly harder to clear and land safely. You don't always have the option to choose another crossing, but in areas where you are stealing one of a type of car instead of one specific car, it might help to steal one nearer the easier bridge to cross.