Cities Skylines has a very loose blanket slope limit of 25%. This might be convenient, but it's not very realistic. I have created a slope limiter mod with the following features:
My mod lowers the limits as follows:
Two lane roads: 15%
Four lane roads: 10%
Six lane roads: 10%
Highways: 7%
Highway ramps: 15%
Railroads: 5%
You can temporary relax these limits by holding LEFT SHIFT while dragging using the road tool. The relaxed limits are as follows:
Two lane roads: 30%
Four lane roads: 20%
Six lane roads: 20%
Highways: 14%
Highway ramps: 30%
Railroads: 10%
This allows for a realistic building experience and some leeway when you are in a pinch.
If you do not like these values, all limits and keys can be configured by modifiying a config file named SmoothSlopesConfig.xml, which is found in (Steam dir)\steamapps\common\Cities_Skylines
This mod has been inspired by (and part of it blatantly copied from) the Stricter Slope Limits mod by Japa. Many thanks to him!
You can find the mod at the Steam Workshop:
http://steamcommunity.com/sharedfiles/filedetails/?id=413311572