Don't give up on the 800 MHz frequencies quite yet. I definitely heard something related to Caltrans this summer north of Santa Maria. I also heard something down in Santa Barbara County also, I was either on the 101 around the Solvang junction, or on the south side of 154 south of San Marcos Pass. Two very isolated receptions, both driving a fair distance in D5. Both times a "5" callsign was heard.
I don't hear much from Caltrans here either, Kirk. We aren't getting any nasty storms and the elevation here is 7,832', in the driveway in front of my garage. The biggest storm this winter at our house has been 6" and anything from 6" down is called a "dusting" here. I drove about 10 miles on partial snowpack on the way to Bishop once this winter and only twice in town for short distances.
Except for the Big Sur to Hearst Castle stretch of Highway 1, the topography in Caltrans D5 is nothing tougher than D9. Sometimes coastal terrain is easy to cover if you have a repeater that "sees" much of the coast. I remember driving back from Cal State SLO in 1970 and picking up both sides of a simplex conversation from the Palos Verdes as I passed Gaviota Beach. This on an 8 channel Regency crystal rig that was about as sensitive as a current handheld scanner with its squelch completely closed and the antenna removed. I had a Shakespeare fiberglass VHF High antenna, but a good antenna can't completely make up for a poor receiver. From the same spot I wouldn't be all that surprised to be able to pick up a handheld with my PSR-600 and rooftop Larsen antenna now. Over water reception past the horizon is pretty good and even if I don't remember the central coast all that well, it would seem as if covering that isolated coastline would not be impossible on 800 MHz. However, my memory is not so good as its been more than 20 years since I've driven that stretch.