Streetmix is a really useful tool for quickly prototyping new layouts for streets. What if you removed vehicle traffic altogether? Or what if you put in a protected cycle path? It also looks great, though the 2D visuals can often be limiting when trying to properly visualise the plans in your head.
Fortunately, Kieran Farr has created a tool that takes a Streetmix link and creates a dynamic 3D visualisation of it, complete with moving vehicles and everything! It really helps to get a better context of what the changes would look like in the real world.
Streetmix3D