Paving The Way For Smoother Roads

Do you remember cruising down the road when you saw that bump at the last second but it was too late to swipe your Starbucks up from the cup holder and hold it in “free space” before you hit that bump—then that frothy, steaming, sticky foaming cup-o-three-dollar-java splashed all over you and that nice, clean dashboard you just shined up on Sunday afternoon? According to a recent Federal Highway Administration (FHWA) survey, a smooth riding road is the number one factor that “most significantly increases public satisfaction with the highway system.” A recent study by The Road Information Program (TRIP) based on data supplied by the Federal Highway Administration (FHWA) 2002 annual survey of pavement condition data, rank California roads second worst in the nation for a smooth ride. The report says that “…Californians pay the highest prices in the country to drive on the worst roads in the nation…”1

Smoother roads last longer, they’re safer, they cause less wear and tear on vehicles, highway maintenance costs are much lower and most importantly, they are much more comfortable to drive on. Have you ever noticed the difference in ride quality when driving I-80 eastbound as you cross the State line entering Nevada? How about I-15 heading from Barstow toward Las Vegas? Or how about driving on I-10 to visit our southern neighbors in Arizona? Nevada and Arizona are among the highest ranking States in the country when it comes to constructing smooth roads. Georgia was recently identified as having the smoothest roads in the country.

As they have learned in Georgia, Arizona, Nevada and many other States, incentives and training that promote the construction of smooth roads create a win-win-win situation for everyone involved. Drivers enjoy the ride quality, lower noise levels and safety advantages that smooth roads provide. Commercial drivers realize lower maintenance and fuel costs on their trucks as well as safer driving conditions. Results from a pavement test track in Nevada showed that trucks could save 4.5 percent of their fuel costs by running on a smoother surface. Government agencies and highway departments benefit from lower maintenance costs, safer roads, extended pavement life and a favorable public image. Contractors can earn bonus pay and be proud of the quality product that they deliver to the end user—the motoring public.

Studies have shown that smoother roads reduce the impact (dynamic) loading forces exerted on a pavement when a tire “jumps” off the road and impacts it again at high speed. Commercial vehicles, tractor-trailer combinations and buses are the most damaging vehicles due to their heavy axle loads. The Kansas Department of Transportation (KDOT) has had a smoothness specification since 1990. KDOT estimates maintenance cost savings from reduced dynamic loadings of $10,000 per mile for the first year and an increase in savings for the life of the pavement. Smoother roads are safer because when vehicle tires hit a bump and leave the roadway surface there is a temporary loss of control when the tire is airborne. Higher speeds greatly increase the risk of losing control as the tires become airborne for a greater distance combined with a reduced force of friction between the pavement and tires. Smooth roads are also safer because of less wear and tear on vehicles’ suspension systems and tires. Repeated loadings cause premature fatigue failure in metal springs, shock absorbers and steel belts in tires, considerably reducing their useful life expectancy. It is estimated that rough roads in the United States cost the nation’s motorists $23 billion a year, which translates to an extra $126 per year in vehicle maintenance costs for the average driver.2 Several studies have also shown that highway maintenance costs are much less when roads are paved to minimum initial smoothness requirements. Some experts estimate that increasing pavement smoothness by 25 percent results in a 10 percent increase in the life of pavements. The Arizona Department of Transportation estimates that their new smoothness requirements will result in a 10 percent increase in the life of their pavements—at no extra cost! Achieving smooth roads requires no new equipment; it’s all in the people doing the work. Training, attention to detail and contractors taking pride in their work is all it takes to place a smooth pavement!

The two most common measuring indices used in the United States today are the Profile Index (PI) and the International Roughness Index (IRI). The PI, originally developed in California for use with the California profilograph, is measured in units of inches/mile (US Units) or millimeters/kilometer (SI Units). A profilograph measures the displacements, in inches or millimeters, from a reference datum as the profilograph traverses the length of the roadway. In simpler terms, it measures the inches of bumps per road mile. The total sum of the vertical displacements is divided by the length of the road over which they are measured and an average PI value is given in inches per mile.

In 1982 the World Bank funded the development of the International Roughness Index (IRI) to provide a way of comparing roadway smoothness information worldwide. IRI is a specific type of profile index. The IRI was developed based on what is known as a “quarter car simulation” (Figure 2).

The intent was to develop a method of data collection that simulated the response of a “typical” vehicle suspension system. It was proposed that this would better approximate the sensation experienced by drivers in their automobiles. IRI values are also reported in units of inches per mile or millimeters per kilometer; however, they do not correlate directly with PI values. A PI = 80 inches per mile is not the same as an IRI = 80 inches per mile. Many States and countries have already adopted the IRI standard. Caltrans has indicated that it will adopt the IRI system in the future.

According to a University of Michigan publication, airport runways and superhighways have IRI values in the range of 10 to 110 inches per mile, while damaged pavements run from 250 to 680 inches per mile. An IRI value of zero indicates a perfectly smooth pavement. Typical IRI specifications for new highway construction range from 60 to 80 inches per mile. The State of Georgia targets 54 inches per mile on new construction—the highest standard in the country. Since implementing smoothness specifications, Georgia has produced smoother roads, contractors are making more money and the State is getting safer, longer lasting pavements at no additional cost.

Laser-based, high-speed profilers that are mounted to pickup trucks are becoming more common for collecting PI and IRI data. They have an onboard computer and one or more lasers mounted on the bumper that record up to 32,000 readings per second as the vehicle drives down the road at the posted speed limit

There are several considerations when paving for smoothness, whether paving with Portland cement concrete or asphalt. Although there are differences between the two types of pavement, some basic guidelines that apply to both are mix consistency (slump and gradation for concrete; gradation, temperature and binder content for asphalt), a continuous paving process, well planned and well executed rolling patterns, and the use of profilographs and profilometers for measuring results. Contractors are very innovative in their approaches to following these general guidelines.


“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 their bid price in anticipation of the money that he will get through incentives at the job’s end.”3

On a recent trip to Ohio a contractor told me that including the anticipated incentive pay for smoothness is common in bidding warranty projects in Ohio. On a recent Ohio Department of Transportation (ODOT) project, a contractor was awarded over $450,000 in incentive pay for smoothness. Pavement smoothness is a tangible, measurable thing that contractors can be innovative in their ideas and equipment used to achieve desired results. The bonus pay to a contractor is not a “give away” of tax dollars. Long-term and accelerated testing studies have proven that a pavement with a higher initial smoothness will last longer and require fewer maintenance dollars for the life of the pavement. The dollars saved in the long-term are more than the dollars paid out as incentives to the contractor. Many specifications are incentive/disincentive specifications, which means that the contractor is penalized for failing to meet minimum requirements, but rewarded for meeting or exceeding target smoothness values. Successful contracts must balance the level of contractor risk with the potential reward for doing exceptional work. Typical incentives for smoothness are a maximum of 5 percent of the contract bid price for the quantity of pavement being placed, however, there are several variations from State to State on how these incentives are written. Some contracts allow up to a 10 percent incentive for meeting or exceeding smoothness requirements.

Caltrans is currently developing a smoothness incentive specification planned for implementation in contracts going out in late 2004 or early 2005. In general, the developing specification considers the initial conditions of the road being paved and the number of opportunities to make improvements to the existing smoothness. An “opportunity” is basically any portion of construction where the existing grade can be made smoother. For example, a straight overlay on an existing road surface is considered one opportunity for improvement. Grinding the existing road surface and then paving is considered two opportunities for improvement, since the road can be made smoother by profile grinding and then again during the addition of the new pavement. As the number of opportunities for improvement increases, superior final smoothness values are expected.

Smoother roads are a win-win-win situation for motorists, owners and contractors. Smoothness is specified by the Owner, achieved by the Contractor and enjoyed by the motoring public. It doesn’t cost any more money to place a smooth pavement. The cost of incentive specifications is far offset by the benefits gained from a smooth riding surface. As the implementation of well-written pavement smoothness specifications become more commonplace, government agencies will realize safer roads at a lower cost, contractors will invent new construction techniques and procedures, and most importantly, YOU, the ultimate customer will get what you’ve been asking for—a smoother ride!

  1. Report by The Road Information Program (TRIP), (a national non-profit research organization) from data supplied by the Federal Highway Administration 2002.
  2. Figures developed by The Road Information Program (TRIP), (a national non-profit research organization) from data supplied by the US Department of Transportation.
  3. Rick Boeger and Roberta J. Crowe., Jan/Feb 2002 Public Roads Magazine.