Have you check the SWR on 17m lately?
How high of the road surface (terra firma) is your feed point?
Bear in mind, bridges has much more rebar plus Ibeam than the road surface.
Match is good. Feed point is about 30 inches above ground. I have not really listened on another band to see if it does it there. Mobile is only place I can do 17 so I tend to use that band mobile. Yes, the rebar and metal in bridge may indeed be a factor here.