High speed road monitor
The high speed road monitor (HRM) measures parameters such as riding
quality, texture and rutting. It consists of a van and 4.5 metre-long trailer fitted
with four laser sensors along the nearside wheelpath. It allows the condition of
the highway’s surface to be assessed under conditions of normal traffic with
speeds up to 95 km/h.
HRM is not designed to replace other slower visual techniques, but will allow
them to be more effectively used by targeting them on the more vulnerable
sections of pavement as identified by this methodology.
The four lasers within the HRM provide data on the longitudinal profile and
macrotexture of the pavement. In addition, a laser mounted at the mid-point of
the trailer axle is used along with the trailer wheels to measure the average rut
depth in the nearside and offside wheelpaths. Inclinometers mounted on the
trailer estimate the crossfall and gradient on the highway. Horizontal radius of
curvature can also be estimated. A device fitted to the nearside trailer wheel pro-
vides distance information to which all HRM measurements are referenced.
The following major parameters are measured within HRM:
Longitudinal profile – profile unevenness is used to assess riding quality for
the road user. It measures deviations away from a moving datum which
represents an average profile along the highway
Rutting – HRM records the average rut depth to the nearside and offside
wheelpaths
Macrotexture – this is the coarser element of the surfacing, formed either by
the aggregate particles in the surfacing or grooves in the concrete surface. It
contributes to skidding resistance, particularly at high speeds, mainly by
providing drainage paths for the water
Alignment parameters – gradient, crossfall and radius of curvature mea-
surements are estimated to an accuracy of±10%.
HRM surveys are only beginning to be used on a routine basis. Results from
them should be used for general guidance only. Its main function is to identify
specific discrete locations where more established but more costly and time con-
suming methods of analysis should be used.
Deflection beam/deflectograph
The deflection beam is a widely accepted instrument for assessing the structural
condition of flexible highway surfacings. Originated by Dr A.C. Benkleman and
developed by the Transport Research Laboratory (Kennedy et al., 1978a), it
involves applying a load to the pavement’s surface and monitoring its conse-
quent vertical deflection. As a loaded wheel passes over the pavement surface,
the deflection of the slab is measured by the rotation of a long pivoted beam
touching the surface at the point where the deflection is to be determined. The
Pavement Maintenance 255