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A close look at road surfaces.


To most people, including the majority of readers of Public Roads, road surfaces are just gray areas stretching for miles and miles. Road surfaces are expected to provide safe driving conditions in dry and wet weather, provide a smooth and quiet ride all the time, minimize splash and spray during rain, provide good visibility under adverse conditions, and have a long service life.

A close look at the surface reveals many features including texture, which is needed to provide skid resistance, reduce splash and spray in heavy rain, and reduce headlight glare in night driving. But texture may increase noise and reduce the life of both pavement and tire. Further, as roads age and deteriorate from the effect of heavy truck traffic and weather, signs of distress appear. Road roughness is one sign of distress and is detrimental to both pavement life and ride quality. This article discusses only road roughness, how roughness is measured, and the effect of roughness on the highway user and on pavement life.

Effects of Roughness

Roughness causes a number of problems to the highway user, including poor ride quality, unsafe driving conditions, excitation of truck dynamics leading to further pavement deterioration, and damage to vehicles and cargo. The vast majority of highway users is most sensitive to ride quality; therefore, ride quality is the primary criterion in setting pavement rehabilitation priorities. Since it is not possible to build perfectly smooth pavements, paving specifications usually prescribe the maximum acceptable roughness.

Measuring Ride Quality

Two basic aproaches to measuring ride quality are currently used. One measures the effect of roughness on ride quality through rating panels or equipment correlated with rating panels. The second approach, called profiling, describes pavement surfaces independent of the measuring equipment.

Ride quality can be determined by pavement serviceability ratings (PSRs) given by panels of drivers and passengers who ride over sections of highways in passenger cars. The PSRs range from zero to five; five represents a perfectly smooth ride. Because of the expense, rating panels are usually limited to a relatively small number - generally about 10 to 20 people. To ensure rating accuracy, the panels are instructed in advance about what to rate and what to ignore. Studies have explored issues of panel rating reliability and accuracy across panels and states, for various pavements, and over time. In one study, a small but significant difference was found between the ratings of two panels from different states in the rating of 31 pavements.[1] In another study, no significant differences were found in two ratings conducted five years apart.[2] A relatively simple way to estimate ride quality by objective means is to measure the dynamic response of a passenger car as it is driven over a pavement. The Federal Highway Administration's predecessor, the Bureau of Public Roads, developed a single wheel trailer - the roughometer - to perform this measurement. Commercial ride meters, installed in passenger cars or trailers, were later introduced.[3] Most Response Type Road Roughness Measuring (RTRRM) systems measure the accumulated suspension deflections over the length of the test sections. The results are expressed as the ratio between suspension deflections in meters (or in inches) and the length of the test section in kilometers (or miles). This roughness index of m/km or in/mi has been in use for many years. The International Roughness Index (IRI), now in common use, has the same units of measure. The standard IRI is derived from a computer simulation using a set of standard suspension parameters and a recorded road profile to drive the simulation.[4] The Pavement Serviceability Index (PSI) computed from the m/km statistics is an estimate of PSR, the panel rating. A statistical relationship and IRI has been developed under a current study.[5]

PSR = 5/exp(C*IRI).

The value of C is 0.226 for flexible pavements and 0.286 for rigid and composite pavements. IRI is in m/km. If the IRI is given in in/mi, it must be divided by 62.6 to convert it to m/km.

Profiling Road Surfaces

Road-surface profiling is another means of measuring road roughness. Road-surface profiles present a "profile," or picture, of the road described in terms of wavelengths and amplitudes.

The road surface profile is measured by road-profiling systems, or profilers for short. Attempts to measure the pavement profile go back to the 1920s. These early profilers, however, lacked an independent reference, and the measurements were therefore affected by the geometry of the profiler.

In 1964, General Motors built a profiler using accelerometers to establish an inertial reference.[6] The inertial reference is used to correct for the bounce of the survey vehicle. This makes it possible to measure true pavement profiles over a wide range of roughness wave-lengths. The recorded profile is independent of the type of survey vehicle and of the profiling speed. This system is commercially available under the trade name "Profilometer."

Between 1974 and 1987, FHWA built two prototypes of profiling equipment, combining longitudinal and transverse profiling. The first, named SIRST (System for Inventorying Road Surface Topography), used 12 infrared sensors to cover the full lane width.[7] It was too costly to be promoted for wide use. The second profiler, named PRORUT (Profile and Rut Depth Measuring System), uses only three sensors.[8] Two sensors measure the profile in each of the wheel tracks; the third sensor measures an average rut depth. PRORUT was evaluated and used successfully by a number of state highway agencies.[9] The cost for building a similar system was estimated to be between $100,000 and $150,000. A new, upgraded PRORUT is now being built in the Pavement Performance Laboratory at the Turner-Fairbank Highway Research Center in McLean, VA. It is similar in layout to the first PRORUT, but uses state-of-the-art data acquisition and computer equipment.

The major cost items of profiling systems are the optical non-contact sensors used for measuring the vertical distance to the pavement. A profiler using ultrasonic instead of optical sensors was built by South Dakota. The cost of such sensors is about 1/20 or less of the cost of optical sensors, enabling South Dakota to build a profiler for less than $50,000.[10] This substitution of sensors reduces the vertical resolution and limits the number of samples per unit length. However, for many applications the performance with ultrasonic sensors is adequate. South Dakota offered assistance to any state interested in building a South Dakota-type profiler.

Currently, there are about 40 South Dakota-type systems in operation. Because of the wide interest and in order to promote use of profiling instead of response-type measurements, the Road Profiler Users Group was formed in 1989 and has met annually.[11] The 1993 meeting is scheduled to be held in Pennsylvania. For more information, contact Gaylord Cumberledge at (717) 787-1199.

FHWA has initiated a pooled fund study entitled "Interpretation of Road Roughness Profiles." The objective is to develop relationships between longitudinal pavement profiles and ride quality, pavement performance, dynamic loads, highway safety, and vehicle and cargo wear. This study is expected to show that different ranges of the pavement profile (different wavelengths) are associated with the various effects outlined in the objective. As an example, it was found in a recent study that panel ratings of ride quality in passenger cars correlate best with an pavement index computed from wavelengths between 0.5 and 2.5 m (1.6 and 8 ft) corresponding to vibration frequencies of 10 to 50 Hz at a speed of 85 km/h (about 53 mi/h).[1]

The pooled fund study is coordinated by the University of Michigan Transportation Research Institute. UMTRI is responsible for developing the relationships mentioned above. Some of these relationships will be empirical, while others will have to be developed from computer simulations, using profile data as one of the inputs. A combination of these two, empirical and simulation, will probably yield the best relationships.

The participating state highway agencies agreed to conduct tests and provide the data and related information to UMTRI. Most states are known to operate South Dakota-type profilers with limited resolution and sampling rates. These limitations may affect some of the planned analyses. Some profilers with better resolution, including FHWA's PRORUT, will be available for this study when more detailed profiles are needed.

A real road-surface profile contains many wavelengths and amplitudes. The larger amplitudes are generally associated with longer wavelengths. Figure 1 shows a record of a typical bituminous road surface over a distance of 100 m (330 ft). The maximum amplitudes here are about 50 mm (2 in). When the long waves are removed by filtering down to 10 m (33 ft), the same surface looks quite different, and the maximum amplitudes are now about half. (See Figure 2).

Another way of presenting the road profile is by the so-called spectrum. (See Figure 3). The horizontal axis shows cycles per unit distance (cycles/m or cycles/ft) which is the inverse of wavelength L. The amplitudes corresponding to each wavelengh are plotted on the vertical axis and are seen to decrease with decreasing wavelength (increasing spacial frequencies).

Ride quality, however, is better discussed in terms of frequency of vibration because humans are more sensitive to some frequencies than to others. Frequency is obtained by dividing the speed of travel by the wavelength, giving lower frequencies for longer wavelengths. Car suspensions are tuned to reduce the vibration amplitudes at the frequencies to which humans are most sensitive. The two curves are similar but are from two different roads. The distribution of wavelengths is about the same, but the amplitudes of one road are greater than for the other one. Ride quality gets worse as the amplitudes increase.

Truck suspensions are designed to support heavy loads and are much stiffer than passenger car suspensions. This is a compromise between the desire to provide good ride quality to truck drivers and carrying full loads without excessive suspension deflection. This results in larger dynamic forces damaging the pavement, the truck, and the cargo. Dynamic forces also lead to momentary reductions of the wheel to pavement contact force, which can lead to reduced traction and loss of control.

At this time, the principal use of the profile is to compute and report IRI values to the Highway Performance Monitoring System. This system is the responsibility of the Office of Highway Information Management. The IRI and other information are compiled in an annual report on the state of the highway system. Before the wide-spread changeover to profiling, the IRI was derived from response-type measurements. The IRI computed from profiles is more reliable and will lead to an overall improved performance measurement.

Some profilers can measure rutting of flexible pavement and faulting of rigid pavements. Ruts are depressions in the transverse pavement profile. Some measuring systems record transverse profiles. A profiler with the two sensors in the rutted wheel tracks and one or more additional sensors can measure rut depth directly. The precision needed for rut depth measurement is not as high as for profiling, so ultrasonic sensors are adequate.

Faults can be measured at travel speeds from profile records, provided the profiler is capable of a high sampling rate. Figure 4 shows a profile recorded by PRORUT at a rate of 10 samples per 0.3 m (10/ft). Faulting is clear from this record, and the fault steps are about 10 mm (3/8 in).


The pavement surface shows many features. Although some are needed for good performance, most develop from exposure to traffic and the elements and are manifestations of distress. Pavement distress affects the highway user in many ways. Ride quality is the most obvious and is a primary factor in pavement management decisions. The highway engineer needs reliable tools to determine the condition of the highway system. Early detection of distress can help to take preventive actions instead of more expensive repairs later on. Profiling systems are very effective in measuring road roughness at travel speeds, and if properly equipped can also measure rut depth and faulting.

Description of the new PRORUT

The new PRORUT is housed in a minivan. The principal components and features are:

* Three laser height sensors which are mounted to a transverse beam below the vehicle body. Two sensors are aligned with the wheel tracks 1.8 m (6 ft) apart. The third sensor s mounted along the vehicle center line. Make and model: SELCOM Optocator Type 2008 Vertical resolution: 0.25 mm (0.01 in). Standoff distance: 390 mm (15.5 in). Linear range: 128 mm (5 in).

* Laser control unit, mounted in rear of van. Make and model: SELCOM Rack with power supply, three receiver averaging boards.

* Power requirements: 1110 VAC, 50-60 Hz. Output: Digital and analog Sampling rate: Up to 16,000 Hz.

* Accelerometers, mounted on left and right height sensors. Make and model: Schaevitz LSB, +/- 2g.

* Computer, mounted on stand and accessible to operator and driver. Make and model: DOLCH Portable 486/33 VGA Color Monitor. Data acquisition board, installed in computer: Data Translation DT 2801A.

* Power supply: 110 V, 1200 Watt. Make and model: Heart Interface HF 12-1200U.

* PRORUT sample spacing: 10 mm or larger (up to 30 samples/ft).

* Test speed 25 to 100 km/h (15 to 60 ml/h).

* Outputs: Raw and processed data, graphical and numerical, choice of filters.

* Statistics: IRI for each wheel track at selected section lengths and average of both wheel tracks. Average rut depth for section length.


[1] M.S. Janoff, J.B. Nick, and P.S. Davit. "Pavement Roughness and Rideability," National Cooperative Highway Research Program Report 275, Transportation Research Board, Washington, D.C., 1985. [2] E.B. Spangler and W.J. Kelly. "Long-Term Time Stability of Pavement Ride Quality Data," Publication No. FHWA/OH-91/001, Ohio Department of Transportation, Columbus, Ohio, 1990. [3] T.D. Gillespie, M.W. Sayers, and L. Segel. "Calibration of Response-Type Road Roughness Measuring Systems," National Cooperative Highway Research Program Report 228, Transportation Research Board, Washington, D.C., 1980. [4] M.W. Sayers, T.D. Gillespie, and W.D. Paterson. "Guidelines for Conducting and Calibrating Road Roughness Measurements," World Bank Technical Paper Number 46, World Bank, Washington, D.C., 1986. [5] B. al-Omari and M.I. Darter. "Relationship between IRI and Pavement Condition," Department of Civil Engineering, University of Illinois, Urbana, Illinois, 1992. [6] E.B. Spangler and W.J. Kelley. "GMR Road Profilometer - A Method for Measuring Road Profile," Highway Research Record 121, Transportation Research Board, Washington, D.C., 1966. [7] J. Derwin King and Stephen A. Cerwin. "System for Inventorying Road Surface Topography (SIRST)," Publication No. FHWA-RD-82-062, Federal Highway Administration, Washington, D.C. 1982. [8] T.D. Gillespie, M.W. Sayers, and M.R. Hagan. "Methodology for Road Roughness Profiling and Rut Depth Measurement," Publication No. FHWA-RD-87-042, Federal Highway Administration, Washington, D.C., 1987. [9] K. Ksaibati, K. Kercher, Sedat Gulen, and T.D. White. "The PRORUT Evaluation in Indiana," Transportation Research Record 1260, Transportation Research Board, Washington, D.C., 1990. [10] D.L. Huft, Debra C. Corcoran, Blair A. Lunde, and Paul A. Orth. "Status of the South Dakota Profilometer," Transportation Research Record 1117, Transportation Research Board, Washington, D.C., 1987. [11] D.L. Huft, D. Boiling, and R. McQuiston. "South Dakota Road Profiler Users' Group," Report of the Meeting, Wyoming Department of Transportation, Cheyenne, Wyoming, 1990.

Rudolph R. Hegmon is a mechanical engineer in the Pavement Division of the Office of Engineering and Highway Operations at the Federal Highway Administration's Turner-Fairbank Highway Research Center. He has worked for FHWA in the areas of traffic safety and truck ride quality since 1973. Dr Hegmon currently is working on the development of instrumentation for the measurement of pavement performance. His research responsibilities include pavement-vehicle interactions and their effect on traffic safety, dynamic axle loads, and ride quality. He manages the Pavement Performance Laboratory at TFHRC.
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Title Annotation:includes related article on upgraded Profile and Rut Depth Measuring System
Author:Hegmon, Rudolph R.
Publication:Public Roads
Date:Jun 22, 1993
Previous Article:75 years old and going strong.
Next Article:Highway, bridge, and transit conditions and performance: 1993.

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