Instruction Manual, Type 9195E - WIM

Typical Configuration of a WIM Station
8.3 Spacing between Two Lineas ® WIM Sensor Rows
The spacing between two Lineas sensor rows depends
mainly on the speed of the vehicles. The main body oscillation
frequency is between 1,8 and 3,5 Hz, we recommend
from experience that the spacing should be within 3 to
5 m.
In practice, there are good reasons to take into account,
not only theoretical calculations, but also experience, for
instance:
The frequency spectrum strongly depends on the vehicle.
Air suspension vehicles are nearer to 1 Hz than
2,5 Hz for body oscillations and their damping characteristics
are different from steel suspension types.
Full or partial loading of vehicles severely affects the
oscillation spectra, and there may be remarkable
differences according to various vehicle populations per
road and country.
Further to body vibrations, the axle hop and tire natural
frequencies are higher and important too.
The overall accuracy is potentially influenced more by
pavement unevenness than by a spacing of 5 m instead
of 4 m. Even the slightest bumps many meters ahead of
the sensors may give rise to oscillation, so the flatness
between sensor and surrounding pavement is essential.
An important practical aspect is that signal integration
requires the individual velocities between the two
sensor rows for each vehicle. Thus the longer the
spacing, the higher the errors caused by acceleration or
deceleration, namely at lower speeds.
Therefore, in view of the many above influences and
practical experience, we refrain from giving you deviation
estimates for other spacing distances.
Expected average velocity Recommended distance d
45 - 75 km/h (30 to 45 mph) 3,5 m
75 - 95 km/h (45 to 60 mph) 4,0 m
>95 km/h (>60 mph) 4,5 m
002-300e-07.04 (200-348e) Page 23