Beware! Bug report incoming! :P
While merrily driving my way through the Elmer challenge, suddenly all road props stopped appearing, leaving just the cars (mine and others') and buildings, but nothing else. Hud was still visible & working, and the timer/counter were behaving as expected (timer going down, counter not raising).
Honking, taking on/off the googles and shooting changed nothing.
Leaving the road solved the glitch.