Toyota PRIUS Manual page 376

2-4. Using other driving systems
 Hill-start assist control
P. 380
 PCS (Pre-Crash Safety System) (if equipped)
P. 383
 Emergency brake signal (if equipped)
When the brakes are applied suddenly, the stop lights automati-
cally flash to alert the vehicle behind.
When the TRC/VSC/ABS systems are operating
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PRIUS PHV_OM_OM47831E_(EE)
The slip indicator light will flash
while the TRC/VSC/ABS sys-
tems are operating.