The performance of flexible and rigid pavements is known to be closely related to properties of the base, subbase, and/or subgrade. However, some recent research studies indicate that the performance predicted by this methodology shows a low sensitivity to the properties of underlying layers and does not always reflect the extent of the anticipated effect, so the procedures contained in the American Association of State Highway and Transportation Officials’ (AASHTO’s) design guidance need to be evaluated...

NCHRP Web-Only Document 261: Case Studies of Truck Activity Data for Emissions Modeling consists of seven case studies that are appendices A to G of NCHRP Report 909: Guide to Truck Activity Data for Emissions Modeling. NCHRP Research Report 909 explores methods, procedures, and data sets needed to capture commercial vehicle activity, vehicle characteristics, and operations to assist in estimating and forecasting criteria pollutants, air toxics, and greenhouse gas emissions from goods and services moveme...

TRB’s National Cooperative Highway Research Program (NCHRP) Web-Only Document 260: Proposed Refinements to Design Procedures for Geosynthetic Reinforced Soil (GRS) Structures in AASHTO LRFD Bridge Design Specifications explores the effect of adopting a closely‐spaced reinforcement layout in geosynthetic‐reinforced soil structures. While research since the early 1980s has identified the beneficial effect of closely‐spaced reinforcement in reinforced soil structures, such improvement in performance is not ...

TRB’s National Cooperative Highway Research Program (NCHRP) Web-Only Document 259: Guidelines to Improve the Quality of Element-Level Bridge Inspection Data is intended to assist inspectors in identifying defects and assigning the appropriate condition state for bridge elements. To ensure the safety and serviceability of bridges, the guidelines include accuracy requirements designed to help promote consistency in the collection of element-level data for bridges on the National Highway System. The quality...

TRB's National Cooperative Highway Research Program (NCHRP) Web-Only Document 258: Manual on Subsurface Investigations provides an update to the American Association of State Highway Transportation Officials (AASHTO) 1988 manual of the same name. This report reflects the changes in the approaches and methods used for geotechnical site characterization that the geotechnical community has developed and adopted in the past thirty years. The updated manual provides information and guidelines for planning and...

TRB’s National Cooperative Highway Research Program (NCHRP) Web-Only Document 256: Assessing Interactions Between Access Management Treatments and Multimodal Users describes operational and safety relationships between access management techniques and the automobile, pedestrian, bicycle, public transit, and truck modes. This contractor's report may help assist in the selection of alternative access management techniques based on the safety and operation performance of each affected travel mode.The r...

TRB's National Cooperative Highway Research Program (NCHRP) Web-Only Document 248: Research on Creating and Sustaining a Culture of Innovation for Departments of Transportation documents the research process and provides key guidance to implement the research produced in NCHRP Research Report 885 : Guide to Creating and Sustaining a Culture of Innovation for Departments of Transportation. This guide provides insight on encouraging and sustaining a culture of innovation within the organization, its partne...

TRB's National Cooperative Highway Research Program (NCHRP) Web-Only Document 257: A Mechanistic–Empirical Model for Top–Down Cracking of Asphalt Pavements Layers develops a calibrated mechanistic-empirical (ME) model for predicting the load-related top-down cracking in the asphalt layer of flexible pavements. Recent studies have determined that some load-related fatigue cracks in asphalt pavement layers can be initiated at the pavement surface and propagate downward through the asphalt layer. However, t...

TRB's National Cooperative Highway Research Program (NCHRP) Web-Only Document 254: Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 2: Training Manual provides training on the underwater placement of filters, and can be adapted into a workshop or distance learning presentations. The project provides guidance on design procedures, material testing requirements, installation alternatives, and quality checklist items for both granular and geotextile filters. Filters ar...

TRB's National Cooperative Highway Research Program (NCHRP) Web-Only Document 254: Underwater Installation of Filter Systems for Scour and Erosion Countermeasures, Volume 1: Research Report documents the research effort of NCHRP Research Report 887 : Guidance for Underwater Installation of Filter Systems. The project provides guidance on design procedures, material testing requirements, installation alternatives, and quality checklist items for both granular and geotextile filters. Filters are an importa...