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.
gaming.stackexchange.com/users/4027/JohnoBoy
I'm afraid not.
The best suggestion I can offer is to enter coop with a friend who hasn't progressed to that point yet, steal the car and deliver it.
gaming.stackexchange.com/users/7864/Shadur
I know an easier way.
The only way I've been doing it for the slow cars and the big ones.
Use the high way.
It's easy and even though it takes longer to get to your destination, you do get there.
There's two highways you can use.
The one that goes across Zombieland and the one down near the Saints HQ.
Try it, I know it won't fail.
Plus, it's easier when you save up on your police forgiveness.
I only ever use the police forgiveness on big trucks and slow cars.
gaming.stackexchange.com/users/19268/courtneyyy.
This may seem like a long shot.
But if you completely 100 percent the game then the bridges should lower.
This means you need to have all collectibles, challenges, vehicle thefts, assassinations, hoods, etc.
gaming.stackexchange.com/users/23082/Steven Y
There is a relatively simple way to do this, although it involves a mod (and admittedly, I wrote it).
If you go to the Saints Row Mods forum there is a simple mod that I created for precisely this reason; it checks on start up whether or not you've completed the full storyline and if you have, it drops the drawbridges, restores the statue and cleans up the zombies.
There is no achievement at the end of the game to lower the drawbridges despite earlier posts.
It just doesn't happen.
This is for several reasons as it turns out; the first is that many of the drawbridges make up the stunt jumps that the player can find.
These are not attached to any challenge or the like, so unless you're a 'collector'
there's no benefit to finding these.
Also, one of the final survival (dial in) missions is a zombie cleanup mission, and just in case you haven't done it by the end of the story they leave the zombies in Arapice Island, quarantined by the drawbridges.
If (like me)
you just want the drawbridges down and don't like zombies (or some variation of this combination)
take a look at the mod, read carefully what happens (you have to finish 3 Way both times for instance) and then install and use at your own risk of course.
Best suggestion is to hang around and wait for the zombie survival mission to come through before starting 3 way so that you can close out your area at 100% complete, but all other challenges that are in the Saints Book section can be completed through alternate sources.
I should also note that there is a much more comprehensive mod (Things to do in Steelport) that allows you to toggle drawbridges and zombies as you desire, as well as does a large number of other things.
If however you just want a lightweight mod that covers this specific function, then follow the link above.
gaming.stackexchange.com/users/202951/Tim B   II
Just replay the final mission and do the alternate ending.
You should get an achievement and the bridges will go down.
However, you cant get them back up once you've done this
so if you intend to complete all challenges, you should complete the big air challenge before getting them lowered
gaming.stackexchange.com/users/24070/Tom B.
At a certain point in the game, the bridges between islands in Steelport raise up.  I'm trying to finish all of the vehicle theft challenges in the Saintsbook, and this makes it a bit more challenging.  Have you ever tried jumping a garbage truck or a limo off a bridge ramp without ending up at the bottom of a river?  Let me tell you, it is  frustrating .   Is there any way to get the bridges to lower after I've finished the game?
Is there any way to lower the bridges?
gaming.stackexchange.com
<saints-row-the-third>
gaming.stackexchange.com/questions/42493
gaming.stackexchange.com/users/13845/agent86
If you complete the game then the bridges should lower.  While the community begs to differ regarding that matter, few options are offered, such as the use of third party software.
A negative answer, regarding the question. It is implied that bridges are supposed to lower if the player completes specific objectives. Use of modifications is suggested.