If a vehicle has a headway smaller than a pre-determined threshold
, it is in the emergency regime. In this case the
vehicle uses an appropriate deceleration rate to avoid collision and
extend its headway. Let 
and 
be the longitudinal positions of the two vehicles (position is
measured as the distance from the downstream end of the segment,
see Figure 3.3); 
the length of the leading vehicle; 
the gap distance from the leading vehicle; and 
the acceleration rate of the leading vehicle. The acceleration rate
for the following vehicle n is given by:
which guarantees that the following vehicle will always decelerate to extend the headway toward a safe range.
