The deflection beam is used on short lengths of highway of 1km or less.
The deflectograph provides a quicker method of assessment and is more suitable for the assessment of long sections of highway. It operates on the same
principle as the deflection beam, but the measurements are made using transducers instead of dial gauges and all measurements are made in pairs, one in
each of the wheel tracks. Loading is provided by two beams mounted on an
assembly attached to the vehicle. An operating cable connects the deflection
beam system to the lorry (Position A in Fig. 9.2). As the vehicle moves, the cable
is let out at such a rate that the beam assembly remains stationary on the
highway allowing the deflection under the rear wheels to be estimated (Position