score:9
Yes, making a U-turn without entering Canada is possible. That being said, it is not easy to do; here is what you would go through if your experience is like mine.
Upon arriving to the Customs, you have to ask them to do a U-turn, then they summon a person who is authorized to allow you to make one. Then, you have to fill out a form that says, among other things, that you have not purchased from the Duty-Free store. After this, you can be approved for a U-turn. You are directed to a gate to wait at; and after your form gets processed, a bridge worker comes out, opens the gate for you, and tells you which path to take in order to complete your U-turn. If you accidentally take a wrong turn, you end up at some restricted location (I just turned around without looking what exactly it was). Not a very short procedure, but if you really must, you can.
Upvote:5
While @Alex's answer confirms that it is possible, evidence from Street View suggests it might not be a good idea - doing this (especially for sightseeing purposes) is clearly bending the rules and could conceivably get you yelled at if the border guards are having a bad day.
Here are the three approaches to the bridge from the US side:
Coming in on I-75 from the North [i.e., southbound] you need to follow direction signs at Exit 47 C ,peel off on the right and follow the lanes into the customs compound merged with traffic from the south.
... but if you follow this exit you get to this fork:
where the right fork is "no re-entry" and the left fork (47C) appears to take you back on to I-75 going the other way ... maybe the rules were looser in 2014 when the original post was written ... ? The border guards might indeed let you make a U-turn to get back into the US (they're not bound by what the highway signs say), but it would be inadvisable to try it unless you have a backup plan. (For the record, this article referenced in comments points out that (as of 2014) there was a bridge employee whose job was to facilitate the return of people who were stuck in the restricted area (e.g. unable to enter Canada because of insufficient documentation or carrying restricted items) get back on the highway on the US side.