> Can't speak for California. In PA more roads and lanes often do help.

 
Pennsylvania does not exist on a fundamentally different plane of existence than California, so no they don't. As any trip through the freeways in Philly will show you.
 
> but not building new roads / adding lanes isn't the answer either.

 
The answer is to remove vehicles from the road. That is the only way to alleviate traffic. Unfortunately, most Americans cannot imagine a life that doesn't involve driving alone an average of 40 miles a day.
 
> In my view, among the most promising is on-demand public transit. Instead of full-size buses running on a fixed routes, using a mix of vehicles from vans to full-size buses instead with flexible routes. Closest analogy would be Uber Pool. If done right, many would use it.

 
Congratulations, you just reinvented the jitney.