Vertical accelerations of a passenger in an automobile are automatically summed while traveling a section of road at 51.5 mph. A roughness index is obtained by summing the accelerations and dividing by the time elapsed during the test. The index is subjectively related to the riding quality of pavements. Continuity in measurements since 1957 has been preserved through correlations among successive vehicles involved and reference pavements.

In general, bituminous construction has yielded smoother-riding surfaces than concrete construction. No major improvements in construction were noted on bituminous pavements since 1962. The ride quality of concrete pavements, however, has improved on those projects where slip-form paving was used. A pavement constructed in 1968 with continuous reinforcement and slip-form paving exhibited excellent ride quality and may be indicative of the results from similar construction in the future. Interstate and parkway construction continues to yield smoother pavements than other major construction. These comparisons, of course, are valid only for the same speed of travel since the tests were conducted at 51.5 mph. However, ride comfort was found to be foremostly related to vehicle speed, and when consideration was given to driving speed, 70 mph on interstate and parkway roads and 60 mph on other highways, the ride quality became significantly reduced on the higher-speed facilities and greatly offset cited improvements.

Bituminous overlaying of the older surfaces has eliminated most of the very rough pavements. As a result of these resurfacing efforts, a reasonable valid claim may be made that the ride quality on most primary, two-lane highways in Kentucky has materially improved since 1957 in spite of the ongoing deterioration of pavements with age, increased traffic and vehicle loads.

The rate of deterioration in ride quality was found to be different for each pavement type and varied according to the original or as-constructed roughness of the pavement, structural number, and the type of highway facility involved. Concrete pavements on interstate and parkway roads deteriorated at a considerably lower rate than bituminous pavements on the same facilities. On bituminous interstate pavements, ride quality deteriorated more rapidly on the smoother-constructed surfaces, while on concrete interstate pavements the rougher surfaces deteriorated more rapidly. Completely opposite trends, however, were realized on the parkway projects. For a given highway facility involving bituminous construction, the lower original roughness indexes were associated with those projects where the structural numbers were higher. A definite trend in increased deterioration in ride quality was noted for concrete pavements as the structural number decreased.

The correlation between roughness index and service age, cumulative traffic and EAL's yielded equally valid statistical results. The contribution of traffic and loading to roughness could not be properly ascertained. Each of the parameters were time dependent and correlated well with each other.

Pavements involving high-type construction generally do not exhibit rapid changes in ride quality. The level of service provided by these highways in regard to ride quality, therefore, are foremostly related to the as-constructed roughness of the pavement.

Report Date


Report Number

No. 316

Digital Object Identifier



The opinions, findings, and conclusions In this report are not necessarily those of the Department of Highways or the Federal Highway Administration.