Skip to contentUnited States Department of Transportation - Federal Highway Administration FHWA Home
Research Home   |   Pavements Home
Report
This report is an archived publication and may contain dated technical, contact, and link information
Publication Number: FHWA-RD-03-041

Evaluation and Analysis of LTPP Pavement Layer Thickness Data

Previous | Table of Contents | Next

This chapter summarizes the results from the evaluation of the thickness data variability indicators based on core thickness measurements and field elevation measurements (SPS only). Typical LTPP layer thickness variability values are summarized by different layer and material types.

The chapter also presents the summary of the comparisons of layer thickness variances and means obtained based on the core and elevation thickness measurements for newly constructed SPS sections for different layer types, material types, and target thicknesses.

Thickness Data Sources

Layer thickness summary statistics such as average, minimum, maximum, standard deviation, and coefficient of variation (COV) serve as indicators of layer thickness variability along the section. For GPS sections, most of these values could be obtained from the LTPP database tables INV_LAYER and RHB_LAYER. These summary statistics were provided by the highway agencies and could be either estimated or computed. No additional information on how summary statistics were derived for these tables is available. For the SPS sections, layer thickness summary statistics could be obtained from the SPS*_LAYER tables. These values were computed from the elevation shots measurements. The SPS*_LAYER tables do not contain summary information on the number of data points used to derive the statistics. No information is available on whether all these data points were used to compute summary statistics or whether some "outlier" points were excluded.

Due to limited information on how the layer thickness summary statistic measures provided in the INV_LAYER, RHB_LAYER, and SPS*_LAYER tables were developed, it was not possible to determine whether statistical indices available in these tables were obtained using similar procedures and whether a comparable number of samples were used to derive the statistical indices. Based on this limitation, no cross-table comparison of layer thickness variability indicators available in these tables was carried out in this study

.

Alternatively, layer thickness summary statistics could be computed using LTPP layer thickness data obtained from individual core measurements or from elevation measurements. The following data sources are available in the LTPP database:

Figure 12 shows schematically where core samples and elevation layer thickness measurements were obtained along the LTPP sections. Core data were obtained for both GPS and SPS sections, while elevation measurements were obtained only for the newly constructed SPS sections.

Figure 12 in page 42 presents a top view (i.e., bird eye view) of a test section from which core samples and elevation layer thickness measurements were taken. Core samples were taken from the 15-meter strips from the beginning and end points of a standard test section that is 182 meters long. Meanwhile, the elevation measurements were taken from the middle 152 meters of the standard test section less the front and end 15-meter strips.
Figure 12: Graph. Location of core sampling and elevation measurement areas along the LTPP section.

Evaluation Methodology for Thickness Variability Reasonableness

Data Assessment and Exclusion of Erroneous Data Points

Two different data sources were used in the analysis of layer thickness variability reasonableness:

Core elevation measurements are available for both GPS and SPS sections, while elevation measurements are available only for the SPS sections. Analysis of layer thickness variability reasonableness was carried out separately for each data source, and the results of analysis obtained from different sources then were compared.

Prior to the statistical analysis, erroneous layer thicknesses measurements were identified and excluded. Several different error sources were identified in the course of this study. Details of erroneous data evaluation are included in the discussion of analyses carried out using data from each data source.

Thickness Variability Indicators

To compare the thickness information at a layer level in lieu of individual measurement level, the following summary statistics from individual measurements were computed for each pavement layer:

COV provides a good measure of whether the dispersion of layer thickness values around the established mean thickness value is large or small. The COV is computed as a ratio between standard deviation and the mean thickness value.

Figure 13 in page 43 shows the equation for coefficient of variation (COV), which is equal to standard deviation (s) of pavement layer thickness data divided by the mean pavement layer thickness (x bar).
Where:
COV= coefficient of variation of layer thickness.
s= standard deviation of layer thickness.
mean layer thickness= mean layer thickness.

Figure 13: Equation. Definition of coefficient of variation.

Thickness Variability Reasonableness Criteria

Criteria established under an LTPP material study [31] were adopted to evaluate the reasonableness of the thickness variability measures, as following:

Evaluation of the Layer Thickness Variation Reasonableness Using Core Data

The analysis is based on evaluation of the layer thickness variation reasonableness for individual LTPP sections and individual layers within the section. Under the LTPP material study [31], the core thickness data for individual layers from the LTPP tables TST_AC01_LAYER and TST_PC06 were evaluated to exclude erroneous data points and to compute summary statistics. These summary statistics were used in this study to evaluate reasonableness of the layer thickness variability indicators for individual layers.

Prior to the analysis, LTPP sections and individual layers with computed summary statistics were correlated with data elements in the TST_L05B table describing experiment, layer, and material types.

The criteria established in the referenced study [31] were used to evaluate the reasonableness of layer thickness variability indicators for each layer that had data in either the TST_AC01_LAYER or TST_PC06 table and in the TST_L05B table. The results of the layer thickness variability evaluation are presented in table 19 for different LTPP experiments, layers, and material types.

Table 19. Summary of project-level layer thickness variability evaluation using core data.
Layer Type Experiment Number of Sections Percentage of Sections with Acceptable Layer Thickness Variations
With Data With COV > 20 % With SD > 8 mm
PCC OverlayGPS-924   7 70.8
SPS-729   10 65.5
PCC Original SurfaceGPS-3126   22 82.5
GPS-461   12 80.3
GPS-584   9 89.3
GPS-743   6 86.0
GPS-924   5 79.2
SPS-2139   40 71.2
SPS-650   1 98.0
SPS-730   5 83.3
SPS-82   0 100.0
SPS-918   1 94.4
LCSPS-235   7 80.0
AC Original SurfaceGPS-1229 9   94.3
GPS-2139 9   93.5
GPS-6143 21   85.3
SPS-1134 2   98.5
SPS-3252 39   84.5
SPS-5133 14   89.5
SPS-818 0   100.0
SPS-925 1   96.0
AC BinderGPS-1147 3   98.0
GPS-283 6   92.8
GPS-32 0   100.0
GPS-6125 20   84.0
GPS-741 8   80.5
GPS_92 1   50.0
SPS-1110 8   92.7
SPS-3118 16   86.4
SPS-5150 22   85.3
SPS-611 1   90.9
SPS-811 0   100.0
SPS-919 1   94.7
AC OverlayGPS-6204 25   87.7
GPS-757 4   93.0
SPS-16 1   83.3
SPS-351 11   78.4
SPS-596 6   93.8
SPS-620 3   85.0
SPS-87 0   100.0
ATBGPS-12 0   100.0
GPS-252 1   98.1
GPS-37 1   85.7
GPS-41 0   100.0
GPS-520 1   95.0
GPS-68 1   87.5
SPS-1102 15   85.3
SPS-324 3   87.5
SPS-513 0   100.0
Total3227 257 125 88.2

Core Thickness Data Availability and Assessment for Newly Constructed SPS Layers

For the newly constructed SPS layers with a documented target thickness, thickness measurements are available from both core examination and elevation measurements. Layer thickness summary statistics computed for the newly constructed SPS layers were compared to the elevation measurements data, as discussed later in this chapter.

To reflect the most recent LTPP data upload status for the newly constructed SPS layers with a specified target thickness, the core thickness data were evaluated again with erroneous data points excluded and summary statistics computed for each layer and each analysis cell. A summary of the available core thickness data for SPS experimental sections is presented in table 20.

Table 20. Core data availability in tables TST_AC01_LAYER and TST-PC06.
Layer Type Experiment Number of Records (measurements) Number of Sections with Data
DGATBSPS-132378
PATBSPS-114232
SPS-200
LCSPS-218236
PCCSPS-2894140
SPS-723522
SPS-8162
SBSPS-1759170
SPS-545592
SPS-69926
SPS-813718
Total 3242616

Using the three-standard deviation criterion, one core thickness record was identified as erroneous (Section 22-0708, PCC layer) and was eliminated from the analysis at the project level. The measured core thicknesses for this layer are between 140 mm (5.5 in) and 149 mm (5.85 in), except for the excluded core measurement that was 198 mm (7.8 in).

Evaluation of the Layer Thickness Variation Reasonableness Using Elevation Data

For SPS newly constructed layers, elevation measurements were taken throughout the section of the final finished surface. The measurements normally are made at five offset points at 152-m (500-ft) spacing along the section.

This big number of elevation thickness measurements available at each layer level makes them a good candidate for thickness variability evaluation. One additional advantage of these thickness measurements is that their layer design or target thickness is known to the research team. As a result, the thickness variability values can be compared and summarized for different target values.

Elevation Data Availability

The availability of elevation data in SPS*_LAYER_THICKNESS tables by layer type and number of sections are presented in table 21.

Table 21. Summary of the elevation thickness measurements in the SPS*_LAYER_THICKNESS tables.
Layer Type Experiment Number of Records (measurements) Number of Sections with Data
DGABSPS-1529597
SPS-2405085
SPS-8 1863 38
DGATB SPS-1 5250 97
PATB SPS-1 4496 83
SPS-2 2242 47
LC SPS-2 2242 47
PCC SPS-2 6955 140
SPS-7 918 24
SPS-8 763 14
SB SPS-1 9138 167
SPS-5 4856 93
SPS-6 1933 40
SPS-8 1202 24
Total  51419 997

The total number of records at Level E in the SPS*_LAYER_THICKNESS tables was 51,419 at the time of the study.

Exclusion of the Erroneous Data Points

Prior to the data analysis, 78 erroneous data points were excluded before the analysis because of data inconsistency. The following list summarizes data inconsistencies found during review of the data from the SPS*_LAYER_THICKNESS tables:

These erroneous thickness values were reported to the FHWA for further investigation.

Additionally, data points that deviated by more than three standard deviations from the mean were considered as potentially erroneous and were excluded from the analysis data set. Analysis of sections with outliers revealed that most of these sections had one outlier per section; some had two outliers, and a few three or four outliers. In all, 202 data points were excluded from further analysis. The summary of outlier analysis is presented in the table 22. A total of 51,139 records were used in the statistical analysis.

Table 22. The distribution of the elevation thickness records not used in the analysis.
Number of Outliers per Layer Number of Layers Total
With Outliers With Other Excluded Points
11625167
215318
3224
41 1
6 11
55 11
Total number of layers18012192
Total number of outlier records20278280

The number of outliers summarized by different layer types is presented in table 23.

Table 23. Distribution of the outliers by layer type.
Layer Type Number of Records (Measurements) Total Number of Records (Measurements) Percent of Records (Measurements)
DGAB 46112080.41
DGATB 1852500.34
PATB 2367380.34
LC 824580.33
PCC 3586360.41
SB 72171290.42
Total 202514190.39

The highest percentage of the sections with outliers is for AC and PCC surface layers and unbound base, while the lowest percentage is for LC base, PATB, and DGATB.

Analysis of Layer Thickness Variation

Elevation measurements obtained after each layer construction were used to conduct analysis of layer variation reasonableness. Table 24 provides summary of the layer thickness variation reasonableness evaluation results for all SPS sections.

Table 24. Summary of project-level layer thickness variability evaluation using elevation grid data.
Layer Type Experiment Number of Sections Percentage of Sections with Acceptable Layer Thickness Variations
With Data With COV > 20 % With SD > 8 mm
DGABSPS-1975 94.8
SPS-2842 97.6
SPS-8383 92.1
DGATBSPS-1970 100.0
PATBSPS-1831 98.8
SPS-2460 100.0
LCSPS-248 2645.8
PCCSPS-2139 6156.1
SPS-724 1441.7
SPS-814 1214.3
SBSPS-11672 98.8
SPS-59212 87.0
SPS-6360 100.0
SPS-8241 95.8

For all material types except for PCC and LC the percentage of acceptable data is very close to or above 90 percent. For PCC and LC material types this percentage is below 60.

Typical LTPP Layer Thickness Variability Values

To estimate typical values for layer thickness variability indicators, layer thickness data for SPS experimental sections were obtained from TST_AC01_LAYER and TST_PC06 tables (core thickness), and from SPS*_LAYER_THICKNESS tables (elevation thickness). The analyses were done separately for the thickness data obtained from core measurements and for the data from elevation measurements. Table 25 summarizes layer thickness COV and standard deviations by layer and material types obtained for PCC and AC layers from GPS and SPS sections based on the analysis of core thickness data. Table 26 summarizes layer thickness COV and standard deviations by layer and material types obtained for the newly constructed SPS sections based on analysis of elevation measurements. The COV and standard deviation values from the tables 25 and 26 could be used as approximate estimates of the expected layer thickness variability along the project for a given material and layer type.

Table 25. Summary of layer thickness COV and standard deviations based on core measurements.
Experiment Type Description Number of Analysis Layers Mean COV, % Min COV, % Max COV, % Mean St. dev., mm Min St. dev., mm Max St. dev., mm
GPSAC Binder39610.100.7883.197.460.87110.28
DGATB886.831.0246.928.341.3061.38
AC Surface5069.760.7093.245.440.52107.46
AC Overlay25910.681.4859.925.440.8744.90
SPSAC Binder38210.410.6271.387.891.2795.19
ATB13912.660.85184.8814.791.47135.97
AC Surface48810.210.6964.285.341.1445.58
AC Overlay16010.700.7270.714.901.1425.85
GPSPCC3362.360.4010.925.441.0431.14
PCC Overlay242.920.5513.106.221.0420.74
SPSLC344.621.1223.387.371.8038.80
PCC2332.660.5127.976.311.1465.21
PCC Overlay295.191.6112.597.222.1914.63


Table 26. Summary of layer thickness COV and standard deviations based on SPS elevation measurements.
Material Type Number of Analysis Layers Mean COV, % Min COV, % MaxCOV, % Mean St. Dev., mm Min St. Dev., mm Max St. Dev., mm
DGAB2198.781.9037.4413.003.2055.76
DGATB975.311.7915.109.503.8724.48
LC485.692.5520.338.963.8132.38
PATB1298.743.4521.218.913.5920.41
PCC1774.180.9817.988.612.8822.96
SB3198.322.0135.808.412.4721.10

Comparison between Elevation and Core Thickness Measurements

For the newly constructed SPS layers (layers that were constructed during the LTPP program and were monitored by the LTPP team), both elevation and core thickness measurements are available in the LTPP database. These two measurement methods employ different measuring techniques. The objective of this section is to evaluate if the means and the variances derived from these two methods are significantly different from each other at the project-level. Thus, the analysis is based on evaluation of statistical indicators derived for each layer of each SPS section. Only newly constructed SPS layers were used in the analysis.

Analysis Methodology

The normality of distribution of elevation data was tested and it was concluded that for a majority of sections and for all material types the distribution is normal. The detailed results are presented in chapter 5. In this analysis it was assumed that core thickness measurements have also normal distribution, because they represent different sort of the measurements for the same kind of data.

The variances and means of layer thickness data were obtained for each newly constructed layer from each SPS section from two different data sources, elevation and core thickness measurements, were compared to determine the level of agreement.

Two statistical procedures were utilized to perform the comparison of elevation and core thickness measurements:

Analysis Data Set

Elevation data for bound asphalt and concrete layers were available for 770 individual layers, while core data were available for only 616 layers. However, both elevation and core thickness data were available for only 498 asphalt and concrete layers. For 118 layers, only core data were available and for 272 layers only elevation data were available. Additionally, for 15 layers only one core measurement per layer was available. Therefore, the total number of asphalt and concrete layers used in the analysis was 483. Table 27 presents the summary of data availability.

Table 27. Summary of layers with both elevation and core data available.
Layer Type Experiment Number of Layers with both Elevation and Core Data
DGATBSPS-159
PATBSPS-130
SPS-2-
LCSPS-231
PCCSPS-2123
SPS-715
SPS-82
SBSPS-1134
SPS-560
SPS-615
SPS-814
Total 483

Comparison of the Standard Deviation and COV Values

Figure 16 provides a comparison of the standard deviations computed from core thickness measurements versus standard deviations computed from elevation thickness for all the layers. For the standard deviation values below 10 mm, the standard deviations computed from the core thickness data are lower than the standard deviations computed from the elevation measurements in most cases. However, for standard deviations above 10 mm, the standard deviations from the core data are higher than the standard deviations computed from the elevation measurements for a significant number of cases. For the majority of the elevation data, the standard deviation is below 20 mm.

Overall, 321 layers (66.5 percent) had a standard deviation computed from the elevation measurements higher than the standard deviation computed from the core measurements. Figure 16 indicates that, for a few sections, the variation of core thickness was very high as compared to the elevation-determined thickness. However, the differences between the standard deviations were not statistically significant (99 percent confidence level) for a large majority of the sections.

Figure 16 in page 53 shows the comparison chart of the standard deviation in millimeters (mm) of core thickness and elevation measurements where the x and y axes represent the standard deviation of elevation measurements and core thickness, respectively. With the diagonal line from bottom left corner to the upper right corner of the chart representing equality of the two standard deviations, most of the data points scatter around bottom left end of the diagonal with x-axis ranging between 0 and 80 mm (mostly less than 20 mm) and y-axis ranging between 0 and 80 mm (mostly less than 10 mm).

Figure 16: Chart. Comparison of the standard deviation for core thickness and elevation measurements.

Figure 17 provides a comparison between the COV values computed from the elevation and core thickness data sets. Over 80 percent of the COV values computed using each data set are below 10 percent. However, a small percentage of sections show low COV computed from one data source and high COV computed using the other data source, i.e. high COVs for elevation measurements and low COVs for core thickness measurements for the same section, or vice versa.

Figure 17 in page 53 shows the comparison chart of the coefficient of variation (COV) in percent of core thickness and elevation measurements where the x and y axes represent the COV of elevation measurements and core thickness, respectively. With the diagonal line from bottom left corner to the upper right corner of the chart representing equality of the two COVs, most of the data points scatter around bottom left end of the diagonal with x-axis ranging between 0 and 80 percent (mostly less than 15 percent) and y-axis ranging between 0 and 80 percent (mostly less than 10 percent).

Figure 17: Chart. Comparison of the COV for core thickness and elevation measurements.

Comparison of the Variances

Table 28 presents the results of the comparison of variances. Sections were grouped by material type, experiment number, target thickness, and subbase type. For more than 80 percent of the sections, the differences between variances obtained from elevation and core thickness measurements were not statistically significant (99 percent confidence level). This percentage is even higher for DGATB and LC layers (about 90 percent).

The greatest differences of variance values were observed for PATB and some analysis cells with PCC and SB layers, and the lowest differences were observed for DGATB and LC layers.

Table 28. Comparison of variances (F-test, 99 percent confidence level) obtained from elevation and core thickness measurements.
Material Type EXP. Target Thickness Subbase Variance Total Number of Sections
Equal Unequal
mm in Number of Sections Percent of Sections Number of Sections Percent of Sections
DGATBSPS-11024 1280.0320.015
2038 2592.627.427
30512 1694.115.917
PATBSPS-11024 2273.3826.730
LCSPS-21526 2890.339.731
PCCSPS-22038S11777.3522.722
W23589.7410.339
27911S1568.2731.822
W3587.5512.540
SPS-7763S685.7114.37
1275S450.0450.08
SPS-82038W1100.000.01
27911W1100.000.01
SBSPS-11024S3382.5717.540
W2586.2413.829
1787S3286.5513.537
W2071.4828.628
SPS-5512S2485.7414.328
1275S2371.9938.132
SPS-61024S12100.000.012
2038S133.3266.73
SPS-81024W571.4228.67
1787W571.4228.67
Total    39782.28617.8483
Notes: S - "Strong" subbase (DGATB, LC). W - "Weak" subbase (DGAB, PATB).

Comparison of the Means

The mean layer thicknesses computed from elevations and those computed from core samples were compared using the t-test at a 95 percent confidence level and assuming either equal or unequal variances, based on the F-test results, presented in table 28. The results of the t-tests are presented in table 29.

Table 29. Results of the comparison of means (t-test, 95 % confidence level) for elevation and core thickness measurements.
Material Type Exp. Target Thicknesses Subbase No Significant Difference between Elevation and Core Thickness Significant Difference between Elevation and Core Thickness Total Number of Sections
mm in Number of Sections Percent of Sections Number of Sections Percent of Sections
DGATB SPS-1102 4   9 60.0 6 40.0 15
SPS-1203 8   20 74.1 7 25.9 27
SPS-1305 12   8 47.1 9 52.9 17
LC SPS-2152 6   20 64.5 11 35.5 31
PATB SPS-1102 4   12 40.0 18 60.0 30
PCC SPS-2203 8 S 14 63.6 8 36.4 22
SPS-2W 16 41.0 23 59.0 39
SPS-2279 11 S 10 45.5 12 54.5 22
SPS-2W 16 40.0 24 60.0 40
SPS-776 3 S 3 42.9 4 57.1 7
SPS-7127 5 S 5 62.5 3 37.5 8
SPS-8203 8 W 0 0.0 1 100.0 1
SPS-8279 11 W 1 100.0 0 0.0 1
SB SPS-1102 4 S 19 47.5 21 52.5 40
SPS-1W 10 34.5 19 65.5 29
SPS-1178 7 S 11 29.7 26 70.3 37
SPS-1W 13 46.4 15 53.6 28
SPS-551 2 S 10 35.7 18 64.3 28
SPS-5127 5 S 12 37.5 20 62.5 32
SPS-6102 4 S 9 75.0 3 25.0 12
SPS-6203 8 S 2 66.7 1 33.3 3
SPS-8102 4 W 3 42.9 4 57.1 7
SPS-8178 7 W 4 57.1 3 42.9 7
Total          47.0 256 53.0 483
Notes: S - "Strong" subbase (DGATB, LC). W - "Weak" subbase (DGAB, PATB).

Based on the t-test results, the mean thicknesses computed from the core measurements are not different from those computed from the elevation measurements at a 95 percent confidence level for 227 (47 percent) of all layers analyzed. The opposite is true for the remaining 256 layers analyzed (53 percent).

Figure 18 presents aggregated results of the statistical analysis of the differences between elevation and core thickness measurements. More than 60 percent of the layers with DGATB and LC had no significant difference between elevation and core thickness data. This percentage is about 40 for PATB, PCC, and SB layers.

Figure 18 in page 56 shows the percentages of significant and insignificant differences between core and elevation thickness data at 5 percent risk level in a bar chart where the x and y axes represent the material type and percent of analysis data cells, respectively. For each of the five material types (DGATB, LC, PATB, PCC, and SB) in the x axis, there is a corresponding bar consisting of a lower blank (no significant difference percentage) bar and a upper black (significant difference percentage) bar. For example, the bar for DGATB consists of a 60 percent high lower blank bar and a 40 percent high upper black bar.

Figure 18: Chart. Results of the statistical analysis of differences between elevation and core thickness measurements.

Summary

In this chapter, the layer thickness variability indicators available in the LTPP database were reviewed. A discussion about the limitations of the available data was provided. In addition, new layer thickness variability indicators (mean, range, standard deviation, COV, and variance) were developed based on the core thickness measurements and field elevation measurements (SPS only) from the most recent LTPP database upload (release 11.5 version NT3.0, obtained on June 8, 2001).

Evaluation of Layer Thickness Variability Reasonableness

Using layer thickness summary statistics, reasonableness of the layer thickness variability data was evaluated. The purpose of the analysis was to compare layer thickness variation for each section and each layer with the benchmark layer thickness variability values. The analysis results indicated that over 88 percent of layers have layer thickness variability indicators below the benchmark values.

Additionally, typical values and ranges of layer thickness variability indicators for different layer and material types were computed. These typical values could serve as approximate estimates of the expected layer thickness variability for the project-level analysis and design.

Excessive Variability in Layer Thickness

For the layer thickness data obtained from the core measurements, 257 layers (10.0 percent) from the TST_AC01_LAYER table and 125 layers (18.8 percent) from the TST_PC06 table had excessive variability in the layer thickness data even after outliers were removed.

For the layer thickness data obtained from the elevation measurements, 139 layers (14.1 percent) from the SPS*_LAYER tables had excessive variability in the layer thickness data even after outliers were removed.

No remedial action was taken for the identified records. However, comment codes were assigned in the analysis summary table to the records containing such data. To determine the reasons for excessive variability, individual core samples should be reviewed.

Comparison of Layer Thickness Variability Indicators from Different Data Sources

Statistical comparisons were made between the layer thickness variances and means obtained from the core and elevation thickness measurements. Only data for newly constructed SPS sections were utilized. The results of the analysis are as follows:

Previous | Table of Contents | Next


The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
FHWA
United States Department of Transportation - Federal Highway Administration