Bridge over a river, that you just cant walk over - or bridge over river that goes up to let boats pass underneath, and it is stuck in an upright position - workmen at the site?
Or just a river, meaning you dont have to actually continue the city one one side of the map - its not uncommon to see rivers running down the middle of cities, have a few unreachable footbridges to make it looks more realistic..
Train Crossing that is constantly down, or a tram (i think they are called roadcars or something in america..those things that are on the streets in san fransisco) across the road..
The edge of city? a big hill that runs up to nowhere, but you cant climb to the top and see over it..
If there has been an unexpected event, normally streets are flooded with onlookers, so police barriers blocking off an entire street wouldnt seem out of place
Just a few ideas