Document Library

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.

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.

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.

Roundabouts are widely acknowledged to be effective at reducing travel speeds. These lower speeds are one of the
reasons why roundabouts tend to experience significantly fewer injuries and fatalities as compared to other types of
intersection control. However, there is limited research as to how speeds vary between roundabouts, signalized, and all-
way stop-controlled intersections. To better understand these differences, this study compares travel speeds between
roundabouts, signalized, and all-way stop-controlled intersections in Minnesota. At signalized intersections, speed data
are analyzed separately for vehicles arriving during the red and green phases. Multiple linear regression models are
estimated to examine average speeds and the deceleration rates at the entry approach, as well as average speeds and
acceleration rates on the exit approach. The results show that drivers begin reducing their speeds further upstream of
roundabouts, and speeds are also lower downstream of the roundabout as compared to other intersection types. This
reinforces research on the traffic-calming effects of roundabouts. At signalized intersections, driver deceleration behavior
is dependent on the signal phase during arrival (later, more rapid deceleration during green arrival or earlier, gradual
deceleration during red arrival). In contrast, behavior is comparable to roundabouts at all-way stop-controlled
intersections, though drivers tended to brake nearer to the intersection at these locations. Ultimately, this study provides
insights into how intersection design can influence driver speed selection and achieve target speeds.

This manual describes a pavement design methodology, termed mechanistic-empirical (ME) pavement design. Based on engineering mechanics that have been validated with extensive road test performance data, this methodology provides the highway community with a state-of-the-practice method for the design and analysis of new and rehabilitated pavement structures. The manual guides pavement design engineers in making decisions using the AASHTOWare Pavement ME Design software, which includes an updated local calibration guide that replaces the publication, Guide for the Local Calibration of the Mechanistic-Empirical Pavement Design Guide, First Edition, published in 2010.

Engineers are increasingly turning to Carbon Fiber Reinforced Polymers (CFRPs) to repair aging, corroded steel bridges because conventional methods are heavy, labor-intensive, and time-consuming. While CFRPs offer a high strength-to-weight ratio and corrosion resistance, past research primarily tested ideal, smooth steel surfaces and unrepresentative environments rather than real-world, deteriorated structures.

This study evaluated how corrosion, pitting, hygrothermal aging, and chloride/environmental exposure affect the bond between steel and two types of CFRP (normal and ultra-high modulus) across laboratory tests and field sites in Virginia. It also evaluated fatigue performance and cost-effectiveness.

Departments of transportation (DOTs) manage extensive bridge inventories while facing increasing pressure from aging infrastructure, extreme weather events, and evolving federal requirements for risk-informed asset management. The Federal Highway Administration (FHWA) requires DOTs to develop and maintain Transportation Asset Management Plans (TAMPs) that address risk management for National Highway System (NHS) pavements and bridges. Although federal guidance emphasizes incorporating risk into asset management planning, practices for identifying, prioritizing, and integrating bridge-related risks vary widely among DOTs. These differences reflect variations in geographic hazards, network characteristics, funding constraints, institutional capacity, and data availability, making it difficult to standardize approaches for quantifying and incorporating risk into decision-making.

NCHRP Synthesis 667: Identifying Risks and Incorporating Them into Bridge Asset Management Planning, produced by TRB's National Cooperative Highway Research Program, documents current DOT practices for identifying, evaluating, and incorporating risk into bridge asset management planning and bridge management plans. The report examines the types of risks considered, the organizational and decision-making structures used to address them, the prioritization approaches employed, and the integration of risk-based priorities into bridge and transportation asset management planning. It also reviews practices for monitoring and recalibrating risk-informed decisions and considers how incorporating risk influences funding and resource allocation.

Employees of state departments of transportation (DOTs) face numerous and dynamic hazards while executing their job duties, including work near moving equipment, work near the traveling public, and work during and responding to natural hazard events. In response, DOTs deploy a wide variety of occupational safety training to provide their employees with the knowledge, skills, and abilities to execute their job duties safely. That wide variety includes various topics, various cycles for updating, various modes of delivery, and various metrics for deployment, among other related issues.

NCHRP Synthesis 665: Occupational Safety Training and Knowledge Sharing for State DOT Employees, produced by TRB’s National Cooperative Highway Research Program, examines how state departments of transportation develop, deliver, evaluate, and update occupational safety training programs. It documents agency practices related to training delivery methods, required training by job function and supervisory level, agencywide versus district-specific programs, and the use of safety data and performance metrics to guide training decisions. The synthesis also reviews approaches for onboarding new employees, measuring training effectiveness, gathering employee feedback, developing specialized training, and involving experienced staff in the creation of in-house training materials. In addition, it examines knowledge transfer planning and the extent to which safety training is provided by in-house staff, consultants, or third-party training providers.

Transportation infrastructure—including buried utilities, drainage systems, and culverts—is frequently subjected to significant external stresses arising from existing conditions and new construction, especially in urban environments where deformation and failures can lead to significant operational and safety consequences. Lightweight fills offer a viable strategy to mitigate these stresses by reducing imposed loads. This study investigates the use of lightweight fills, specifically Foamed Glass Aggregate (FGA), as a sustainable solution to support California Senate Bill 1 (SB1) efforts to repair and maintain culverts, as well as for new construction applications. A numerical investigation was conducted to evaluate the performance of culverts backfilled with FGA compared to conventional fills. Models were developed for reinforced concrete box culverts and corrugated steel pipe culverts, and parametric analyses were performed across a range of configurations. Results indicate that FGA significantly reduces soil stresses, structural demand, deformation, and settlement compared to conventional fills. This study also developed design and construction guidance and proposed updates to Caltrans manuals to facilitate implementation in practice. The findings demonstrate that FGA is an effective, sustainable alternative for improving culvert performance in California and beyond, helping agencies better address aging infrastructure and increasing construction demands.

Time allowances and recovery margins in rail scheduling are incorporated into travel time estimation and minimum headways to prevent delays from affecting subsequent trains. These margins act as a buffer against service irregularities, such as variations in station dwell times and train performance, and they are a key factor in rail capacity calculations. Most contemporary tools used to analyze or construct timetables calculate running times specific to each train. However, the factors influencing running times are inherently uncertain, including human behavior, weather conditions, train composition, ongoing infrastructure projects, rolling stock, and route conditions. To accommodate these uncertainties, transit planners and agencies incorporate time allowances into running time estimation, while applying operating or recovery margins to minimum headways to limit delay propagation.

TCRP Research Results Digest 122: Operating Margin and Recovery Factor Practices for Travel Time Estimation and Rail Scheduling, produced by TRB’s Transit Cooperative Research Program, explores the methodologies used by transit agencies to estimate more robust runtimes. These methodologies, among other elements, help establish operating margins and recovery factors in rail scheduling. This research informed updates to Chapter 8 of the TCQSM 4th edition. The digest describes one of 12 small research tasks conducted to support the development of new content for the 4th edition.