This is possible, but you have to use "undocumented" features.
Warning: Since these are not documented features, they probably aren't official and likely subject to change without notice. Use in production at your own risk.
tldr:
https://www.google.com/maps/dir/{origin}/{destination}/data=!4m3!4m2!2m1!2b1
Plug in origin and destination and it should load directions, avoiding toll roads.
Explanation:
Here, we are using Google's own data/options string (see: "data=") to get the desired functionality. It is meant to be used by the Google Map's web app itself to communicate options selected on the page. This article (and the ones following it) explain how to create your own "data=" strings to customize how the map is configured. It's quite comprehensive, but like I said above, its not an official feature.