One well known overpass in my region built to serve 2 lanes but only having 1 is NJ 55 North approaching NJ 42 North.
https://goo.gl/maps/NWg4iBHBvyHTxFBg9In 1985, the overpass opened when NJ 55 opened. NJ 55 had a short accel lane into NJ 42. Then in 1999, NJ 42 was widened to 4 lanes, providing a thru lane for traffic coming in from NJ 55.
The reason why they just used 1 lane on the bridge is there was no real place to put a 5th lane on Rt. 42 to allow 2 thru lanes to continue. The options would be to do what they did - merge NJ 55 down to 1 lane prior to the overpass (where visibility is good), merge NJ 55 down to 1 lane on or after the overpass, where traffic could find themselves hitting congestion on a curve, or merge 2 lanes down to 1 lane after NJ 55 merged into NJ 42. This would probably alleviate some congestion on 55, but increase congestion on 42.
A long-delayed construction project will provide for a 5th lane on NJ 42 across 2 overpasses (before ending as an 'Exit Only' lane for 295 South), and this project will also allow the overpass to finally allow 2 lanes of traffic, as it was designed for when originally built 38 years prior.
BTW, depending on the GSV image you get above, you may see light traffic, congested traffic where the GSV vehicle is always behind a truck, or in one instance, the travel lane is closed, and traffic is using the hash-marked shoulder which will eventually become the 2nd lane.
