  Traffic waves, known also as stop-and-go waves or phantom hams, appear
naturally as traffic instabilities, also in confined environments as a
ring-road. A multi-population traffic is studied on a ring-road, comprised of
drivers with stable and unstable behavior. There exists a critical penetration
rate of stable vehicles above which the system is stable, and under which the
system is unstable. In the latter case, stop-and-go waves appear, provided
enough cars are on the road. The critical penetration rate is explicitly
computable, and, in reasonable situations, a small minority of aggressive
drivers is enough to destabilize an otherwise very stable flow. This is a
source of instability that a single population model would not be able to
explain. Also, the multi-population system can be stable below the critical
penetration rate if the number of cars is sufficiently small. Instability
emerges as the number of cars increases, even if the traffic density remains
the same (i.e. number of cars and road size increase similarly). This shows
that small experiments could lead to deducing imprecise stability conditions.
