Traffic isn't a function of road space. The true throughput of roads is immense when utilized by better drivers (one extreme example: https://www.youtube.com/watch?v=4pbAI40dK0A). A lot of traffic is caused by bad human drivers: accidents (and associated rubbernecking), shockwaves, running red lights, double parking, circling around looking for parking. Additionally entire additional lanes of traffic (for cars or personal mobility vehicles) will be available when streetside parking isn't necessary. Personally I look forward to the possibilities of turning all that unecessary parking space into something useful ("we’re the Saudi Arabia of developable land in cities").