Road Anarchy
This mod is no longer being maintained. See https://steamcommunity.com/sharedfiles/filedetails/?id=802066100&searchtext=Anarchy for a working version.
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NO RULES. Removed most of the rules governing the placement of roads and tracks. No limit to slope, much higher height limit, barely any collision checking, allows you to pagedown past 0 which will cause the road to dig a hole. Also allows you to build roads over zoned buildings without removing them (so long as the pillars don’t touch them).
Warning, buildings roads crazy enough to cause cars to "detrack" will cause your game to crash, possibly lots of other things could crash the game too, use responsibly and keep multiple saves!
Press L to disable or enable.
Source: https://github.com/lxteo/City-Skylines-Roads/tree/master/ToolsAnarchy