My guess is that "filtering right against a red stop light" means "right turn on red (light)".

According to Wikipedia, turning right on a red light (after stopping fully) is now legal in all 50 states as in Canada, except when there is a sign prohibiting it, and in the city of New York and island of Montreal.

In the course of looking this up, I found this site that appears to be by a UKer about driving in the U.S. and Canada. Might be easier to digest than some of this confused and piecemeal discussion:
http://www.johncletheroe.org/usa_can/driving/index.htm.

~ Becky in Long Beach