This follows the Detector Volume dataset published by the city at (http://data.cityofpasadena.net/datastreams/81752/detector-volumes-data-view/). Sensors at each intersection measure the number of cars per 15-minute time window. This replays the data as a visualization, with circles with a radius proportional to the traffic volume.
This also includes markers for traffic collisions, from (http://data.cityofpasadena.net/datastreams/82095/traffic-collision-data-view/), trimmed to overlapping times. Since the actual data sets didn't overlap (collisions were 2012-2013, and the volume was 2/1-2/6 of 2014), this mockup added 1 year to the collision data, for demonstration purposes.