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.

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.

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.

The article describes the Washington State Department of Transportation’s (WSDOT) efforts to implement wildlife crossing structures, which aim to improve habitat connectivity and reduce wildlife-vehicle collisions. It highlights the state’s data-driven prioritization framework, established under the Washington Habitat Connectivity Action Plan, to identify and target high-risk highway segments. Additionally, the text underscores the importance of long-term monitoring, often done in partnership with universities, to document the ecological and safety successes of these structures.

Adverse weather conditions such as rain, snow, fog, and high winds pose significant challenges to roadway safety and mobility. These conditions not only increase the risk of crashes but also contribute to travel delays, congestion, and unpredictable driving behavior. To mitigate these risks, transportation agencies are increasingly deploying variable speed limit (VSL) and real-time warning (RTW) systems, which use real-time data to dynamically adjust speed limits and provide timely hazard alerts.

NCHRP Research Report 1176: Real-Time Warnings and Variable Speed Limits— Safety and Travel Reliability During Weather Events, produced by TRB’s National Cooperative Highway Research Program, is a guide designed to support transportation professionals in understanding, evaluating, and implementing these systems in response to adverse weather conditions. The strategies and case studies included in this guide are intended to help transportation agencies enhance roadway safety, improve traffic flow, and ensure drivers receive accurate, timely information to navigate hazardous conditions effectively.

The same project that produced NCHRP Research Report 1176 also documented its methodology and findings in NCHRP Web-Only Document 447: Evaluating the Impacts of Real-Time Warnings and Variable Speed Limits on Safety and Travel Reliability During Weather Events.

This publication is an official Crash-Stats Brief Statistical Summary issued in July 2026 by the National Highway Traffic Safety Administration's (NHTSA) National Center for Statistics and Analysis. The report's primary purpose is to provide early, near-real-time statistical projections of national, regional, and state-level motor vehicle traffic fatalities and fatality rates per 100 million vehicle miles traveled (VMT) for the first quarter of the year. To generate these early projections before final census data is completed, NHTSA employs a time series cross-section regression methodology that models the relationship between monthly data from the Fatality Analysis Reporting System (FARS), NHTSA’s non-public Early Notification (EN) program, and Monthly Fatality Counts (MFC). These traffic safety estimates are further calculated using VMT projections provided by the Federal Highway Administration (FHWA). Overall, this document serves as a preliminary forecasting instrument designed to rapidly catalog and share safety trends with the broader highway safety community, with the understanding that these early estimates will undergo minor statistical revisions over the next two years as final FARS files are processed.

The Montana Department of Transportation installed nearly 600 miles of sinusoidal centerline rumble strips (SCLRS) in 2021. These were mostly installed on high-speed, two-lane rural highways, in the Kalispell Division and Missoula area. The sinusoidal pattern produces lower exterior noise levels than conventional centerline rumble strips, but the safety effects of the pattern have not been quantified. This study performed an observational before-after analysis of the SCLRS installation on two-lane rural highways using the Empirical Bayes (EB) methodology. Crash modification factors (CMFs) for a variety of crash types were less than 1.0, indicating that SCLRS were associated with a reduction in crash frequency. The CMF associated with total target crashes—defined as opposite direction collisions—was 0.711 while the CMF associated with fatal plus injury target crashes was 0.624. The only CMF that exceeded 1.0 was for the single-vehicle run-off-the-road crash type; however, this result was not statistically significant. This study also compared the safety performance of the SCLRS to that of conventional centerline rumble strips in Montana. Because the installation dates of the conventional pattern were unknown, a causal inference framework was used for evaluation. The results indicate that the CMFs for the conventional and sinusoidal patterns have confidence intervals that overlap, suggesting similar safety performance associated with the two patterns. An economic evaluation found that both centerline rumble strips patterns have a benefit-cost ratio exceeding 1.0.