score:9
In California, Caltrans has a website with estimates of Autos Southbound Border Wait Times for the San Ysidro and Otay Mesa crossings.
BorderTraffic.com has webcams for many crossings. Some are free, while others involve a fee for access.
Upvote:2
A few people on Reddit told me there exist Facebook groups dedicated to the border queuing times, but it's quite messy:
Zach Lipton's pointer to Caltrans is likely better.