Document Library

This report provides practical guidance for more accurate evaluation of traffic congestion costs and potential congestion reduction strategies. It examines key costing factors including analysis perspective and scope, baseline speeds, congestion exposure (the amount of travel that experiences), travel time valuation, additional impacts (fuel, crash and emission costs, plus pedestrian delays and economic productivity), congestion dynamics, equity analysis and research standards. It discusses principles that should be applied for efficient and equitable congestion analysis, and identifies common biases. It critically evaluates various congestion cost studies, and identifies examples of win-win strategies that reduce congestion and help achieve other goals. It identifies a gap between the performance indicators commonly used in congestion cost studies and what decision-makers need for efficient and equitable planning. This report is more comprehensive and current than most previous studies, and includes novel analysis. It is a companion to Congestion Costing Critique and Smart Congestion Relief.

TCRP Research Results Digest 118: Effects of Strategies to Enforce Bus Lanes, produced by TRB's Transit Cooperative Research Program, describes selected bus lane enforcement strategies currently used by transportation agencies that operate bus lanes and reviews the documented effects of these strategies on bus speeds and reliability. TCRP Research Results Digest 118 summarizes one of 12 small research tasks conducted to support the development of new content for the Transit Capacity and Quality of Service Manual, Fourth Edition.

TCRP Research Results Digest 121: Implementing Microtransit Within Broader Network Design, produced by TRB's Transit Cooperative Research Program, explores the state of the practice for planning, implementing, and operating on-demand microtransit services in the United States. The digest highlights trends and lessons learned from case studies and includes recommendations for incorporating these findings into the Transit Capacity and Quality of Service Manual, Fourth Edition. This digest summarizes one of 12 small research tasks conducted to support the development of new content for the Fourth Edition.

When transportation programs receive federal funding, there are typically domestic preference conditions, or “Buy America” requirements, associated with the use of those funds. The Buy America provision associated with Federal Transit Administration (FTA) grant funds is one of the longest-standing federal transportation Buy America provisions and is probably the subject of more documentation than all the others. The Buy America requirements applicable to FTA-assisted procurements have long provided that all steel, iron, and manufactured products be produced in the United States, except where FTA has issued a waiver of the requirements.

TCRP Legal Research Digest 63: Guide to Buy America Requirements for Transit Agencies—2025 Supplement, produced by TRB’s Transit Cooperative Research Program, provides a comprehensive, updated summary of Buy America requirements applicable to FTA-assisted procurements. It incorporates changes since the publication of TCRP Legal Research Digest 49: Updated Guide to Buy America Requirements—2015 Supplement, including the 2021 Build America, Buy America provision, which expanded Buy America requirements for federally assisted infrastructure projects to include construction materials beyond steel and iron. It also addresses subsequent FTA guidance, decisions and waivers, relevant court cases, and legal disputes identified through outreach to FTA grant recipients.

Drivers leave the roadway for many reasons, including inattention, excessive speed, adverse weather, and medical conditions. Roadside hardware, such as guardrails, guard cable, concrete barriers, impact attenuators, breakaway signposts, and light poles, is designed to reduce crash severity by redirecting vehicles, slowing or stopping them, or absorbing impact. Although roadside hardware consistently saves lives and reduces crash severity, it is frequently the subject of litigation. Transportation agencies are primarily responsible for maintaining these systems but depend on designers, manufacturers, testing facilities, and installation contractors to develop, test, and install them properly. Defendants in personal injury or wrongful death claims may include governmental entities, design consultants, installation contractors, testing facilities, product designers, or manufacturers. These parties may be subject to allegations of negligence, including negligent design, installation, maintenance, or testing. Manufacturers may also face products liability claims.

NCHRP Legal Research Digest 96: Liability for Roadside Safety Hardware Claims, produced by TRB’s National Cooperative Highway Research Program, focuses on the legal theories of negligence, breach of contract, and products liability, which are the primary sources of law in this area. The digest explains the legal theories of negligence, breach of contract, and products liability; summarizes significant case law involving roadside safety hardware; and briefly addresses governmental testing and the evaluation of roadside hardware performance.

Drilled shafts are widely used as deep foundations for transportation infrastructure; however, construction defects such as soil intrusion, concrete segregation, slurry inclusions, necking, and soft-bottom conditions can compromise structural integrity and long-term performance. Conventional crosshole sonic logging (CSL) provides valuable quality-control information but offers limited capability for imaging shaft geometry and defect extent. This study developed, optimized, and field-validated a two-dimensional ultrasonic full waveform Inversion (UFWI) framework for high-resolution drilled shaft imaging using CSL waveform data. The method combines acoustic wavefield modeling, adjoint-state inversion, and regularized model updating to reconstruct cross-sectional P-wave velocity distributions without prior knowledge of shaft boundaries or defect locations. Synthetic studies demonstrated that UFWI can accurately image shaft geometry and detect internal and boundary defects. Parametric analyses further showed that at least five CSL access tubes are required for reliable imaging, while approximately one tube per foot of shaft diameter is recommended for shafts 6 ft in diameter or larger.

Field validation was conducted on a full-scale 6-ft-diameter, 40-ft-long drilled shaft constructed at the Florida Department of Transportation (FDOT) Hawthorne test facility with intentionally embedded defects, including concrete segregation, necking with soil intrusion, slurry inclusions, and a soft-bottom condition. UFWI successfully reconstructed shaft boundaries; identified major defect horizons; and localized anomalies consistent with documented as-built conditions. Compared with conventional CSL, UFWI provided higher spatial resolution and more detailed characterization of defect geometry, demonstrating its potential as a quantitative nondestructive evaluation tool for drilled shaft quality assurance and integrity assessment.

This report evaluates through comprehensive laboratory testing the potential of utilizing quarry by-products (QBs) in Otta seal applications as a sustainable and economical alternative to conventional aggregates. An asphalt surface treatment used on low volume roads, Otta seal consists of a relatively thick application of asphalt binder, covered with a graded aggregate layer that allows the use of locally available and marginal-quality materials. Commonly used chip seal design methods were modified to develop a rational design approach for Otta seal, as the conventional Otta seal design procedure is largely empirical. Material characterization was conducted to determine aggregate gradation, physical properties, and emulsion characteristics required for Otta seal design. A double-layer Otta seal configuration was selected based on laboratory optimization involving testing of various configurations and incorporating QBs. The effect of curing temperature and duration on performance was also investigated through weight-loss monitoring and post-curing sweep testing. The performance of laboratory-optimized, QB-based Otta seals was then evaluated and compared with typical Otta seal aggregate through multiple laboratory tests. Performance assessment included sweep testing to evaluate aggregate retention, a modified Hamburg wheel-tracking (HWT) test to evaluate aggregate retention under simulated braking conditions, and close-range photogrammetry to quantify surface texture. The QBbased Otta seal specimens showed performance trends comparable to those constructed with typical Otta seal aggregate. The best-performing design method was selected, and QB-based specimens were further evaluated for susceptibility to bleeding under repeated wheel passes using modified HWT, friction characteristics using a dynamic friction tester, and changes in composite modulus using a box-test setup with lightweight deflectometer testing. Results indicated that QB-based Otta seal
provides adequate resistance to bleeding and sufficient friction for safe traffic movement, demonstrating strong potential as a sustainable and cost-effective pavement-preservation alternative for low-volume roads.

The Federal Railroad Administration, through a research program to improve transportation safety for tank cars, sponsored the Volpe National Transportation Systems Center to develop and compare techniques to simulate the plastic stress-strain behavior of Association of American Railroads’ TC128, Grade B (TC128) steel [1]. TC128 steel is used in the construction of tank cars designed to carry hazardous materials. This research took place between 2019 and 2025.

This article explores how transportation and land-use planners can establish Freight-Efficient Land Uses (FELUs)—land-use patterns designed to minimize both the private costs and external costs associated with moving goods, guided by the findings of NCHRP Research Report 998: Planning Freight-Efficient Land Uses: Methodology, Strategies, and Tools. The article describes the need for FELUs and long-term planning strategies to foster FELUs.

Over the past several decades, the U.S. vehicle fleet has shifted toward larger, taller, and heavier vehicles, raising questions about whether roadside safety systems remain aligned with real-world vehicle characteristics. This study evaluated the adequacy of the Florida Department of Transportation’s current 31-inch guardrail height standard relative to the previous 27¾-inch standard and recent guardrail failure outcomes in Florida. The research included a review of guardrail height practices, analysis of vehicle center of gravity, weight, and fleet composition trends, identification and classification of confirmed guardrail failure crashes, statistical comparison of failure patterns by guardrail height and vehicle characteristics, and an Annual Average Daily Traffic-normalized hot-spot analysis. Confirmed failures with known guardrail height were distributed in proportions consistent with statewide inventory prevalence, indicating no evidence that either height standard was overrepresented in the failure dataset overall. Vehicle center of gravity was significantly higher in Over failures than in Through failures, while vehicle weight was not a significant predictor of failure mode. Over failures were directionally less common at 31-inch installations than expected based on inventory proportions, but the difference was not statistically significant. The hot-spot analysis identified locations where repeated failures may warrant additional field or engineering review. Overall, the findings support continued use of the 31-inch standard while emphasizing continued monitoring of vehicle fleet trends, guardrail performance, site-specific failure conditions, and potential future design adaptations as the vehicle fleet continues to evolve.

TCRP Research Results Digest 120: On-Demand Transit: Quality of Service Measures and Standards, produced by TRB's Transit Cooperative Research Program, examines how transit agencies and other operators across North America assess the performance of on-demand transit services, with a particular focus on QOS. The digest identifies metrics and benchmarks used to evaluate these services and documents the development of the QOS framework for on-demand transit used in the Transit Capacity and Quality of Service Manual (TCQSM), Fourth Edition. TCRP Research Results Digest 120 summarizes one of 12 small research tasks conducted to support the development of new content for the TCQSM, Fourth Edition.