Caltrans (California department of transportation) has provided a limited set of data about routes, annual miles traveled and annual vehicle hours of delay at 35 miles per hour (for the years 2014-2016). In theory, this could help us see the extent of traffic congestion on some routes.
On the first diagram, we see that annual vehicle miles traveled through Tulare look suspicious: in 2015, the number of miles was much higher than in the years before and after. This could be a possible mistake or a signal that a major road was closed and traffic was rerouted through this city. Without further detail, we can't know the reason.
When it comes to annual vehicle hours of delay at 35 miles per hour, we see that (almost exclusively) routes related to Los Angeles seemed to experience the most delays during that period. In 2016, the delay on route 405 has increased significantly.
The last diagram tries combine the information of the first two by relating normalized distance to normalized delay. We can think of it this way: the longer distance we can travel per hour of delay, the better the road fulfills its transportation purpose. This could mean either less congestion or smoother driving due to infrastructural improvements. Here we see that route 46 through Kern seems to have been quite efficient in 2015, followed by route 74, passing through Orange. In 2016, route 12 passing through San Joaquin has started to improve in performance.