9.2 Pavement Evaluation

2 Roughness

	Major Topics on this Page
	2.1	Measurement
	2.2	Measurement Techniques

Pavement roughness is generally defined as an expression of irregularities in the pavement surface that adversely affect the ride quality of a vehicle (and thus the user). Roughness is an important pavement characteristic because it affects not only ride quality but also vehicle delay costs, fuel consumption and maintenance costs. The World Bank found road roughness to be a primary factor in the analyses and trade-offs involving road quality vs. user cost (UMTRI, 1998). Roughness is also referred to as "smoothness" although both terms refer to the same pavement qualities.

More Information on Pavement Roughness

For detailed information, the University of Michigan Transportation Research Institute (UMTRI) maintains an excellent Web page on roughness and all related aspects at http://www.umtri.umich.edu/erd/roughness.

2.1 Measurement

Today, roughness is typically quantified using some form of either present serviceability rating (PSR), international roughness index (IRI) or other index with IRI being most prevalent.

WSDOT Pavement Roughness Measurement

WSDOT uses IRI to quantify pavement roughness in accordance with AASHTO PP 37-02. Specifically, WSDOT measures IRI data in accordance with ASTM E 950 using a 91.44 m (300 ft) wave length filter and uses terminology consistent with ASTM E 867.

2.1.1 Present Serviceability Rating (PSR)

The AASHO Road Test (Highway Research Board, 1962) developed a definition of pavement serviceability, the present serviceability rating (PSR), that is based on individual observation. PSR is defined as "The judgment of an observer as to the current ability of a pavement to serve the traffic it is meant to serve" (Highway Research Board, 1962). To generate the original AASHO Road Test PSR scores, observers rode around the test tracks and rated their ride using the quantitative scale shown in Figure 9.1. This subjective scale ranges from 5 (excellent) to 0 (essentially impassable). Since PSR is based on passenger interpretations of ride quality, it generally reflects road roughness because roughness largely determines ride quality.

Figure 9.1: Individual Present Serviceability Rating Form

2.1.2 International Roughness Index (IRI)

The international roughness index (IRI) was developed by the World Bank in the 1980s (UMTRI, 1998). IRI is used to define a characteristic of the longitudinal profile of a traveled wheeltrack and constitutes a standardized roughness measurement. The commonly recommended units are meters per kilometer (m/km) or millimeters per meter (mm/m). The IRI is based on the average rectified slope (ARS), which is a filtered ratio of a standard vehicle's accumulated suspension motion (in mm, inches, etc.) divided by the distance traveled by the vehicle during the measurement (km, mi, etc.). IRI is then equal to ARS multiplied by 1,000. The open-ended IRI scale is shown in Figure 9.2.

Figure 9.2: IRI Roughness Scale (replotted from Sayers et al., 1986)

2.1.3 Correlations Between PSR and IRI

Various correlations have been developed between PSR and IRI. Two are presented here. One was reported in 1986 by Paterson:

where:	PSR	=	present serviceability rating
	IRI	=	international roughness index

Another correlation was reported in a 1992 Illinois funded study performed by Al-Omari and Darter (1992):

where:	PSR	=	present serviceability rating
	IRI	=	international roughness index

This study used data from the states of Indiana, Louisiana, Michigan, New Mexico, and Ohio for both flexible and rigid pavements. The associated regression statistics are R² = 0.73, SEE = 0.39, and n = 332 sections. Correlations are highly dependent upon the data that are used.

2.2 Measurement Techniques

The equipment for roughness survey data collection can be categorized into the four broad categories shown in Table 9.1.

Table 9.1: Roughness Measuring Equipment

Equipment / Technique	Complexity
Rod and level survey	most simple
Dipstick profiler	most simple
Profilographs	simple
Response type road roughness meters (RTRRMs)	complex
Profiling devices	more complex

The following discussion with a few modifications was taken directly from the "Pavement Condition Data Collection Equipment" article in the FHWA Pavement Notebook (1989).

2.2.1 Survey

A survey (performed by a survey crew) can provide an accurate measurement of the pavement profile. The use of surveys for large projects, however, is impractical and cost prohibitive.

2.2.2 Dipstick Profiler

The dipstick profiler can be used to collect a relatively small quantity of pavement profile measurements. The Dipstick Profiler (see Figures 9.3 and 9.4) consists of an inclinometer enclosed in a case supported by two legs separated by 305 mm (12 in.). Two digital displays are provided, one at each end of the instrument. Each display reads the elevation of the leg at its end relative to the elevation of the other leg. The operator then "walks" the dipstick down a premarked pavement section by alternately pivoting the instrument about each leg. Readings are recorded sequentially as the operator traverses the section. The device records 10 to 15 readings per minute. Software analysis provides a profile accurate to ± 0.127 mm (± 0.005 in.). A strip can be surveyed by a single operator in about one-half the time of a traditional survey crew. The dipstick is commonly used to measure a profile for calibration of more complex instruments.

2.2.3 Profilographs

Profilographs have been available for many years and exist in a variety of different forms, configurations, and brands. Due to their design they are not practical for network condition surveys. Their most common use today is for rigid pavement construction inspection, quality control, and acceptance. The major differences among the various profilographs involve the configuration of the wheels and the operation and measurement procedures of the various devices.

Profilographs have a sensing wheel, mounted to provide for free vertical movement at the center of the frame (see Figure 9.5). The deviation against a reference plane, established from the profilograph frame, is recorded (automatically on some models) on graph paper from the motion of the sensing wheel (see Figure 9.6). Profilographs can detect very slight surface deviations or undulations up to about 6 m (20 ft) in length.


Figure 9.5: Profilograph	Figure 9.6: Profilograph Output

WSDOT Profilograph Information

WSDOT uses the California Profilograph to check rigid pavement construction smoothness (WSDOT Test Method 807). The measured parameter, called a "profile index", was developed by the California Division of Highways starting in 1956. The goal was to develop a relationship between a calculated index and a subjectively obtained panel rating of road roughness. After a careful survey, the profile index was developed based on 0.2 inch "blanking band" as illustrated below.

The blanking band significantly reduced the data analysis effort since only "scallops" (deviations or excursions of roughness above or below zero) that exceeded the blanking band would be significant and have to be analyzed. Scofield recognized that "…the blanking band precluded faulting up to 0.2 inches. Although considered annoying, the vibration caused by this level of faulting did not create accelerations large enough to produce discomfort to the passengers."

Thus, the profile index is a count of the inches per mile in excess of the 0.2 inch blanking band. WSDOT specifies the following profile index parameters:

A daily profile index < 7 inches per mile. This specification attempts to ensure an overall construction smoothness.
High points having deviations > 0.3 inches shall be ground down so that they do not exceed 0.1 inches. This specification prevents a single large bump from being averaged out over a days' worth of data.

Profile index can be roughly related to other measurements of smoothness and condition. One study showed that for every 2 inches per mile increase in profile index, PSI decreases by about 0.1.

Scofield, L.A. (1993). Profilograph Limitations, Correlations, and Calibration Criteria for Effective Performance-Based Specifications. Final Report, Project 20-7, Task 53, National Cooperative Highway Research Program, Transportation Research Board. Washington, D.C.

2.2.4 Response Type Road Roughness Meters (RTRRMs)

The third category of roughness data collection equipment is the response type road roughness meters (RTRRMs), often called "road meters". RTRRM systems are adequate for routine monitoring of a pavement network and providing an overall picture of the condition of the network. The output can provide managers with a general indication of the overall network condition and maintenance needs

RTRRMs measure the vertical movements of the rear axle of an automobile or the axle of a trailer relative to the vehicle frame. The meters are installed in vehicles with a displacement transducer on the body located between the middle of the axle and the body of a passenger car or trailer. The transducer detects small increments of axle movement relative to the vehicle body. The output data consists of a strip chart plot of the actual axle body movement versus the time of travel.

The disadvantage of a RTRRM is that its measured axle body movement vs. time depends on the dynamics of the particular measurement vehicle, which results in two unwanted effects (UMTRI, 1998):

Roughness measuring methods have not been stable with time. Measures made today with road meters cannot be compared with confidence to those made several years ago.
Roughness measurements have not been transportable. Road meter measures made by one system are seldom reproducible by another.

Because of these two effects, profiling devices are becoming more popular.

2.2.5 Profiling Devices

Profiling devices are used to provide accurate, scaled, and complete reproductions of the pavement profile within a certain range. They are available in several forms, and can be used for calibration of RTRRMs. The equipment can become fairly expensive and complex. Three generic types of profiling systems are in use today:

Straight edge. The simplest profiling system is a straight edge. Modifications to the straight edge, such as mounting it on a wheel, result in a profilograph.
Low speed systems. Low speed systems such as the CHLOE profilometer are moving reference planes. The CHLOE is a long trailer that is towed at low speeds of 3 to 8 kph (2 to 5 mph). The slow speed is necessary to prevent any dynamic response measurement during the readings. A few agencies still use the CHLOE to calibrate their RTRRMs.
Inertial reference systems. Most sophisticated road profiling equipment uses the inertial reference system. The profiling device measures and computes longitudinal profile through the creation of an inertial reference by using accelerometers placed on the body of the measuring vehicle to measure the vehicle body motion. The relative displacement between the accelerometer and the pavement profile is measured with either a "contact" or a "non-contact" sensor system.

The earliest profiling devices used a measurement system in direct contact with the pavement to measure profile. Several contact systems have been used, and are still in use today. The French Road Research Laboratory developed the Longitudinal Profile Analyzer (APL) in 1968.

Systems used today in the United States are frequently installed in vans (see Figure 9.7) which contain microcomputers and other data handling and processing instrumentation. Older profiling devices are usually contact systems, while the more recently manufactured devices use non-contact sensors. The non-contact systems use probes, either acoustic or light, to measure differences in the pavement surface. For instance, the South Dakota road profiler simultaneously collects three ultrasonic profiles, one for each wheelpath and one for the lane center. These profiles are used to calculate (by computer) a mathematical measure of roughness and an estimate of rutting at specified intervals along the roadway. A hybridized South Dakota road profiler combines the three ultrasonic sensors with two laser sensors, one for each wheelpath, for simultaneous measurement of the same roadway by two different sensor types under identical conditions (Virginia Transportation Research Council, 1996). Integrated analysis units, as pictured in the interactive picture below, can continuously collect a wide variety of data at highway speeds such as:

Transverse profile/rutting
Grade, cross-slope
Pavement texture
Pavement condition or distress

GPS coordinates
Panoramic right-of-way video
Pavement video
Feature location


Figure 9.7: South Dakota Road Profiler (van-mounted)	Integrated Analysis Vehicle Interactive Picture (Click Picture to Launch)

WSDOT Profiling Device

WSDOT makes roughness measurements using laser equipment mounted in their distress data collection van. This van, purchased in 1999, records pavement profile (ride, faulting, and rutting) and digital images of the pavement surface, ahead view, and shoulder view.

WSDOT's Distress Data Collection Van

Prior to 1999, roughness measurements were made with a South Dakota Road Profiler equipped with ultrasonic sensors. WSDOT's switch to the laser measurements caused roughness measurements throughout the State to decrease substantially. These decreases, often in the 0.5 - 1.0 m/km range, were not consistent but did occur on almost all pavements regardless of type. Therefore, extreme caution must be exercised when comparing pre-1999 and post-1999 IRI values in Washington State as a substantial percentage of their difference may be due to the change in measurement equipment and not actual road roughness.

2.2.6 Summary of Measurement Devices

A summary of the most commonly used roughness data collection devices, their measurement principles, relative costs, relative degrees of accuracy, and current and projected future use is contained in Table 9.2.

Table 9.2: Roughness Data Collection Equipment (from FHWA, 1989)

Roughness Data Collection Device	Principle of Measurement	Relative Initial Cost	Relative Data Collection Cost (Network)	Relative Degree of Accuracy	Approximate Decade of Development	Extent of Current Use	Projected Extent of Use
Dipstick	Direct Differential Measurement	Low	Impractical	Very High	1980s	Limited, Used for Calibration	Same as Current Use
Profilographs	Direct Profile Recordation	Low	Impractical	Medium	1960s	Extensive for Const. Acceptance	Same as Current Use
BPR Roughometer	Device Response	Low	Low	Medium	1940s	Limited	None
Mays Meter	Vehicle Response	Low	Low	Medium	1960s	Extensive	Decreasing Continuously
South Dakota Road Profiler	Direct Profile Recordation	Medium	Low	High	1980s	Growing	Rapidly Increasing
Contact Profiling Device	Direct Profile Recordation	High	Medium	Very High	1970s	Limited	Decreasing
Non-Contact Profiling Device	Direct Profile Recordation	High	Medium	Very High	1980s	Medium	Increasing Continuously