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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

It's The Ride That Count$

by Rick Boeger and Roberta J. Crowe

Americans' love affair with the automobile extends beyond the cars they drive. It's the lure of the open highway and a comfortable ride. Nationwide, motorists agree that they like their roads flat and their rides smooth.

According to a recent Federal Highway Administration (FHWA) survey, the roadway surface quality - a smooth ride - is the number one factor that "most significantly increases public satisfaction with the highway system." A smoother ride means a more comfortable ride with less noise and vibration, which are especially noticeable on long trips.

Smoothness is a tangible qualifier. It's something a road engineer or paving contractor can sink his or her teeth into. It can be controlled, mathematically designed, and scientifically measured.

And so, the Maricopa County (Ariz.) Department of Transportation (MCDOT) in Phoenix has put in place a program that makes contractors on roadway paving projects put their money where the ride is. In short, the smoother the ride of a newly constructed road or maintenance overlay project, the more money they can take to the bank. Contractors, under this incentive program, can earn as much as an additional 10 percent of total project paving costs in incentive bonuses by exceeding the preset standard for smoothness. The potential downside is that they are also hit in the pocketbook if they don't meet the standard.

However, contractors in Maricopa County are enthusiastically buying into the program. The program has created a "win-win" scenario. MCDOT receives a superior quality product that has increased longevity and requires less maintenance. The contractor has the opportunity to earn more money. And the ultimate winner is every motorist because he or she gets a smoother and quieter ride, less wear and tear on the vehicle's suspension system, and better gas mileage.

Paving contractors, encouraged by the program of reward or penalty, use more accurate paving equipment and work harder to fine-tune the variables within their control. The contractor can control several variables in process and method. By maintaining the optimal asphalt mix temperature and consistency; using a steady, non-stop paving process; having a consistent methodology; making appropriate onsite equipment modifications; executing well-planned roller strategies; and using profilographs and profilometers for measuring results, a contractor can construct and deliver a better product. It comes down to greater emphasis on innovation, skill, care, and quality control.

Table 1 - Adjustment for Rideability Chart

(inches per mile)

(meters per kilometer)
Percent Adjustment
51 - 60
0.80 - 0.95
61 - 80
0.96 - 1.26
81 - 100
1.27 - 1.58
101 - 110
1.59 - 1.74
111 - 120
1.75 - 1.89
Requires Replacement

Payment to the contractor shall be based on the IRI according to this table. The percent adjustment will be applied to payment(s) for the total quantity of hot-mix asphalt used in travel lanes only upon competition of the final course of pavement.

Paying an incentive to satisfy the public's desire for a smooth road does not mean that it has to cost the taxpayer more money over the life of the roadway because a smoother road requires less maintenance and lasts longer, stretching the public's dollar. Also, a construction contractor, confident in quality outcomes, may lower his bid price in anticipation of the money that he will get through incentives at the job's end.

Bumps in the Road - Evaluation Method

Prior to the placement of the final course of pavement, the engineer/construction manager furnishes the construction contractor with an International Roughness Index (IRI) value that results from the engineer's evaluation of the pavement material placed to date. IRI represents the vertical (upward and downward) displacement a passenger would experience traveling at the posted speed limit in a standard vehicle. The IRI value from this preliminary test serves as a guide to the contractor as he evaluates his level of conformance to the smoothness specification. The contractor will then use the IRI value as the basis for determining any pavement adjustments necessary for the final course. The finished road will be measured, and the contractor will be paid based on the "Adjustment for Rideability" chart (table 1).

The final pavement surface is evaluated by MCDOT's Roadway Management Section for smoothness using a test vehicle with a laser-operated measuring device mounted to the frame. A beam of light is shot between the device and the pavement surface, capturing the exact distance between the precise instrument and the pavement surface at preset and equal intervals over the profile established by the operator. Continuously recorded data are compiled and averaged to obtain the contractor's "rideability" score. A zero IRI value would indicate a perfectly smooth pavement surface, while increasing IRI values would correspond to an increasingly rough pavement surface and a bumpier ride.

The Maricopa County (Ariz.) Department of Transportation in Phoenix indulges America's "love affair" with the open road by developing pavement smoothness standards and using incentives and disincentives in construction contracts to build exceptionally smooth roads.

Case Study - Giving It the Road Test

Following three overlay-paving test projects, MCDOT was ready to implement its Smoothness Specification Program. Based on its success with overlays, the county is considering the use of a one-inch (2.54-centimeter) asphalt rubber surface on all new roadway construction. Asphalt rubber pavement has consistently emerged as superior in smoothness with the added benefit of reduced road noise. Typically, asphalt rubber overlays placed over roadways with varying levels of roughness have provided smoother rides than new road construction with conventional mixes.

The first paving project in which the smoothness incentives/disincentives were used was the asphalt rubber overlay of a section of Maricopa Road southwest of Phoenix in fall 2000. The average IRI of this 17-mile (27-kilometer) "penetration and chip seal" segment before the overlay of the eastbound and westbound lanes was 127 inches per mile (about 2 meters per kilometer) and 129 inches per mile (about 2.035 meters per kilometer), respectively. The average IRI after the overlay of the eastbound and westbound lanes was 54 inches per mile (about 0.85 meters per kilometer).

A non-stop paving operation with a continuous feed of asphalt product to the paving machine is a major factor within a construction contractor's control that contributes to final payment smoothness.

This project was a huge success. The after-overlay IRI is the lowest recorded to date in Maricopa County, and it portends a future of smoother roads throughout the county.

Also, the MCDOT Roadway Management Division did their homework; they expected to keep the incentive costs within a range of 3 percent to 5 percent of the total contract price. For the Maricopa Road project with an estimated cost of $1,965,119, the total paid incentive was $77,217, which is 3.93 percent of the contract price.

The Bottom Line Is a Straight Line

Every aspect and every phase of construction is a new opportunity to achieve a smoother, flatter roadway surface. With smoothness specifications and monetary incentives, quality control and industrial innovation are paramount. Construction contractors and public works agencies nationwide will benefit from new construction techniques, processes, and procedures. Roadway maintenance time will decline, lessening inconvenience for motorists. A cost savings will result. However, best of all, our ultimate customer, gets what he asked for - a smooth ride.

Experienced equipment operators, the right equipment, and the timing of the finish rolling are critical.


Paving for smoothness is helped by innovative ideas. The combination of an extended paving ski with auto adjust, a string line, and a heated ski trailing behind the paving machine contributes to accuracy of the lay down of the surface course of pavement.


Rick Boeger heads the Road Management Section of the Maricopa County (Ariz.) Department of Transportation's Construction and Operations Division. Since 1994, he has overseen the MCDOT road maintenance network. Boeger joined MCDOT in 1985, and his tenure has included assignments in highway materials testing and inspection, maintenance planning and budgeting, roadway evaluation, and smoothness specifications. He is also a member of the Transportation Improvement Program Review Committee and the Palo Verde Nuclear Generating Station emergency response team. He has a bachelor's degree in environmental sciences from Western Illinois University and is certified with the National Institute for Certification in Engineering Technology (NICET) Level III, Highway Materials.

Roberta J. Crowe, as part of MCDOT's Community and Government Relations Division, oversees public relations and public participation on all MCDOT projects. She is also the RightRoads program architect and manager. She has more than 15 years experience in public and media relations in the public, private, and non-profit sectors. She has a bachelor's degree in mass communications with emphases in journalism and public relations, and she has completed some post-graduate studies in marketing, advertising, and public involvement from the National Transit Institute at Rutgers, the State University of New Jersey. She is a member of the International Association for Public Participation, the City-County Communications and Marketing Association, and Women in Transportation. Crowe has received national awards for public involvement and for excellence in county government.