The decision support system for the maintenance of paved roads, PMS, of the
Swedish National Road Administration comprises a number of models for
estimating the costs of road management authorities, road users and society
at large. In 1998 VTI was commissioned by the Road Administration to carry
out a comprehensive project with the objective of revising and augmenting
these traffic effect models. The project is entitled "Traffic effects of the
road surface" and has comprised a number of subprojects. The primary aim of
this study has been to investigate, in a field study, the relationship
between the roughness of the road surface (expressed in terms of the
International Roughness Index IRI, with the units mm/m) and the perceived
driving comfort of motorists, as well as their willingness to pay for
improved driving comfort. 50 test subjects were asked to drive a car over
nine 500 m long sections with different surface standards. The mean of the
measured IRI on these road sections varies from 0.8 mm/m on the smoothest
section to 10.5 mm/m on the roughest. In conjunction with the investigation,
the test subjects were interviewed by a test leader. During the interview,
they were asked which factors are significant for driving comfort and how
they perceived driving comfort on the different sections. The state/condition
of the asphalt surfacing was considered to have the greatest significance,
followed by the car and the behaviour of other road users. Density of traffic
was considered to have the least significance. The test subjects were also
asked to say what significance different defects in the surfacing had on
driving comfort. What is most important for a comfortable car journey is that
there are no holes/depressions in the surfacing. This is followed by ruts,
roughness that causes vibrations, and bumps and subsidence. The test subjects
were asked to make a general assessment of driving comfort on a five-degree
scale during their drive over the nine sections. A clear relationship was
found between the assessment that the test subjects made and the measured
IRI, i.e. the higher the IRI, the worse driving comfort was judged to be.
The physical impact caused by the different surface standards of the test
sections was also investigated by placing a dummy equipped with triaxial
accelerometers in the passenger seat of the car that drove over the test
sections. The willingness of the test subjects to pay for improved comfort
associated with the condition of the road surface was also investigated by
three methods: Stated Preference (Choice) and Contingent Valuation Method
(driving comfort versus petrol price and trip time). A relatively high
willingness to pay was obtained, but the methods gave very different results.
It has therefore been impossible to posit a comfort cost model on the basis
of these. This requires further studies and development of the methods.