  Traffic flow oscillations, including traffic waves, are a common yet
incompletely understood feature of congested traffic. Possible mechanisms
include traffic flow instabilities, indifference regions or finite human
perception thresholds (action points), and external acceleration noise.
However, the relative importance of these factors in a given situation remains
unclear. We bring light into this question by adding external noise and action
points to the Intelligent Driver Model and other car-following models thereby
obtaining a minimal model containing all three oscillation mechanisms. We show
analytically that even in the subcritical regime of linearly stable flow (order
parameter $\epsilon<0$), external white noise leads to spatiotemporal speed
correlations "anticipating" the waves of the linearly unstable regime.
Sufficiently far away from the threshold, the amplitude scales with
$(-\epsilon)^{-0.5}$. By means of simulations and comparisons with experimental
car platoons and bicycle traffic, we show that external noise and indifference
regions with action points have essentially equivalent effects. Furthermore,
flow instabilities dominate the oscillations on freeways while external noise
or action points prevail at low desired speeds such as vehicular city or
bicycle traffic. For bicycle traffic, noise can lead to fully developed waves
even for single-file traffic in the subcritical regime.
