Living in California I have Ezpass for Chicago ;)
Going there every 2-3 years I figured out it is worth the time saving.
In Bay Area I use FasTrack, what is the same technology. Having 1 transpoder for the family, I just lay it on the dashboard and it works just fine.
In the past I tried to hold it against the windshield and I think my fingertips on the side blocked the signal as it did not activate.
Still licence plate will do.
Not really advertised, but seem all the companies allow adding licence plate AFTER violation. Saved my frinds couple of times.