RBBM
My musings only . . .
She's a straight-A student, which means pressure; getting into a top college, maintaining her GPA, studying for her SATS.
I can understand the need to get away and escape; I'm thinking long bike rides help her cope with said pressure. My BIL does the same thing; he bikes all over the place. It's good therapy, per se.
Did she fight with anyone at home? If so, did she leave her phone behind on purpose?
I keep coming back to the lateness of the day and possibly getting hit--by a motorist, e-bike or whatever. Or just not seeing well in the dusk; if she went North and into the hills, the lengthening shadows from hills, trees, etc., may have caused her to have an accident.
Praying she is found soon.
ETA: I've enclosed a link to Hill Drive and what this street looks like if one goes North. I think this gives an idea to the narrower streets and sharp turns.
Street View · Google Maps
JMVHO.