Date of Graduation


Document Type


Degree Type



Statler College of Engineering and Mineral Resources


Civil and Environmental Engineering

Committee Chair

Yoojung Yoon

Committee Member

Dimitra Pyrialakou

Committee Member

Fei Dai


The international roughness index (IRI) was established in the 1980s and later used in generating the present serviceability index (PSI), which was developed to predict present serviceability rating (PSR). PSR is a measure of ride quality ratings made by a panel of highway users. The ratings range from 0 to 5, where “0” for very poor and “5” for very good. Various studies had formulated the relationship between IRI and PSI for different types of pavements, such as flexible, rigid, and composite pavements. The current IRI-based PSI equation being used among many state Departments of Transportation (DOTs) was proposed by Al-Omari and Darter in 1994. They analyzed the data collected from six different states: Louisiana, Michigan, New Jersey, New Mexico, Ohio, and Indiana. The use of the equation has been helpful for many state DOTs for general design decisions and pavement management practices, along with other physical conditions and smoothness measures. However, there have been increasing demands in adjusting the PSI equation with disparities between IRI-based design requirements for new pavements and PSI values estimated from IRI. For example, according to the PSR descriptions approved by Federal Highway Administration (FHWA), pavement surfaces below 4.0 begin to show visible signs of surface deterioration. Meanwhile, the acceptable IRI values for new asphalt and concrete pavements range from 52 to 66 inches/mile and from 57 to 72 inches/mile, respectively. The IRI values for asphalts and concretes are equivalent to 4.04 to 3.81 PSI and 3.96 to 3.75 PSI, indicating that most of the new pavements have preservation needs for the year when constructed according to the equation. Therefore, the objective of this study was to calibrate the current IRI-based PSI equation for reliable use in pavement design and management. The primary approach used to calibrate the current PSI equation was comparing the IRI values calculated from the quarter car parameters of the golden car used to develop the current PSI equation and modern passenger cars found from an extensive discovery search. The approach was driven by the hypothesis that there has been improvement in the suspension systems of modern passenger cars that significantly impact the comfort levels of users driving on pavements. The data analysis results presented a newly adjusted PSI equation in the format of the current equation. The calibrated PSI equation was evaluated in terms of three aspects, such as the similarity between the current and calibrated PSI equations, the consistency between IRI requirements for new pavements and PSR definitions, and acceptable ride quality based on IRI. The evaluation found that the calibrated PSI equation is not very distinctive for the first and section evaluation aspects. However, the calibrated PSI equation could be considered to revise the national IRI criterion for acceptable ride quality on national highway systems. Finally, the contributions of this thesis are as follow: 1. It makes clear that the calibrated PSI equation can potentially replace the current PSI equations so that state DOTs can determine acceptable ride quality for national highway systems; 2. It solidifies the research approach as the most cost-effective to determine the current PSI equations; 3. It reinforces the value of the primary research approach for state DOTs looking to adjust quarter-parameters in IRI measurement equipment; and, 4. Provides evaluative criteria that future researchers can utilize to update the current PSI equation.