
Levels of Service for Road Transportation
There are 6 levels of service:
- (A) Free Flow Traffic. Individual users are practically
unaffected by the presence of other vehicles on a road section.
The choice of speed and the maneuverability are free. The level
of comfort is excellent, as the driver needs minimal attention.
- (B) Steady Traffic. The presence of other vehicles on
the section begins to affect the behavior of individual drivers.
The choice of the speed is free, but the maneuverability has somewhat
decreased. The comfort is excellent, as the driver simply needs
to keep an eye on nearby vehicles.
- (C) Steady Traffic but Limited. The presence of other
vehicles affects drivers. The choice of the speed is affected and
maneuvering requires vigilance. The level of comfort decreases quickly
at this level, because the driver has a growing impression of being
caught between other vehicles.
- (D) Steady Traffic at High Density. The speed and the
maneuverability are severely reduced. Low level of comfort for the
driver, as he must constantly avoid collisions with other vehicles.
A slight increase of the traffic risks causing some operational
problems and saturating the network.
- (E) Traffic at Saturation. Low but uniform speed. Maneuverability
is possible only under constraint for another vehicle. The user
is frustrated.
- (F) Congestion. Unstable speed with the formation of
waiting lines at several points. Cycles of stop and departure with
no apparent logic because created by the behavior of drivers. High
level of vigilance is required for the user with practically no
comfort.
The rate of traffic service is the maximal hourly rate that
can cross a point or a road section according to road, traffic and control
conditions. Therefore, each road infrastructure has five traffic rates
of service (the F level is not used because unstable). Traffic reports
also use color codes to illustrate traffic conditions such as green
(levels A and B), yellow (levels C and D) and red (levels E and F).