Toyota Prius Owner's Manual page 516

5-2. Steps to take in an emergency
Warning message
(Flashes
rapidly)
(If equipped)
(If equipped,
flashes)
516
PRIUS_OM_OM47A19E_(EE)
Details
Indicates that there is a high
possibility of a frontal collision,
or that the pre-crash braking
function is operating.
A buzzer also sounds.
At the same time,
will appear on the
head-up display (the image
flashes).
Indicates that your vehicle is
nearing the vehicle ahead (in
radar cruise mode).
A buzzer also sounds.
At the same time,
will appear on the
head-up display (the image
flashes).
Correction
procedure
Slow the vehicle by
applying the
brakes.
Slow the vehicle by
applying the
brakes.