How do I build a bridge when SimCity says "The slope is too steep"?
Solution 1:
When I Googled "The slope is too steep", it linked me to a Reddit post which linked to imgur.
This suggests that you should make your bridge longer. The error can indicate that the slope between the bridge surface and the interchange with the road at the end is too great. If you move the connection further away from the main bridge, it should have more ability to make a gentle slope to the connection.