Welcome to Flyertalk!

CalTrans, the state highway department, has traffic movement sensors embedded in freeway lanes in most urban areas; these can be accessed over the Internet from a variety of sources.

Go to maps.google.com and type in San Clemente, CA (or your starting and ending addresses). Click on Traffic and the live traffic report will be displayed. In your area, Google Maps will display each direction of the I-5 as either red, yellow, or green. Next, switch from "Live Traffic" to, say, "8 AM Monday" and you will get the average traffic readout for any given Monday at 8 AM.

Note that there is about a 2 mile section of yellow in an otherwise-usually-green northbound stripe along the I-5 south of San Clemente, this corresponds to the location of the Border Patrol checkpoint.

Simple as that.