Turner-Fairbank Highway Research Center

The empirical Bayes (EB) method addresses two problems of safety estimation: it increases the precision of estimates beyond what is possible when one is limited to the use of a 2- to 3-year accident history, and it corrects for the regression-to-mean bias. The increase in precision is important when the usual estimate is too imprecise to be useful. The elimination of the regression-to-mean bias is important whenever the accident history of the entity is in some way connected with the reason why its safety is estimated. The theory of the EB method is well developed. It is now used in the Interactive Highway Safety Design Model and will be used in the Comprehensive Highway Safety Improvement Model. The time has come for the EB method to be the standard and staple of professional practice. The study’s goal is to facilitate the transition from theory into practice.

The potential benefits of geosynthetic reinforced soil foundations are investigated using large-scale model footing load tests. A total of 34 load tests were performed to evaluate the effects of single and multiple layers of geosynthetic reinforcement placed below shallow spread footings. Two different geosynthetics are evaluated: a stiff biaxial geogrid and a geocell. Parameters of the testing program include the number of reinforcement layers, spacing between reinforcement layers, the depth to the first reinforcement layer, plan area of the reinforcement, the type of reinforcement, and soil density. Test results indicate that the use of geosynthetic reinforced soil foundations may increase the ultimate bearing capacity of shallow spread footings by a factor of 2.5.

Cooperative adaptive cruise control (CACC) includes multiple concepts of communication-enabled vehicle following and speed control. Definitions and classifications are presented to help clarify the distinctions between types of automated vehicle-following control that are often conflated with each other. A distinction is made between vehicle-to-vehicle (V2V) CACC, based on vehicle–vehicle cooperation, and infrastructure-to-vehicle CACC, in which the infrastructure provides information or guidance to the CACC system (such as the target set speed value). In V2V CACC, communication provides enhanced information so that vehicles can follow their predecessors with higher accuracy, faster response, and shorter gaps; the result would be enhanced traffic flow stability and possibly improved safety. A further distinction is made between CACC, which uses constant-time-gap vehicle following (forming CACC strings), and automated platooning, which uses tightly coupled, constant-clearance, vehicle-following strategies. Although adaptive cruise control (ACC) and CACC are examples of Level 1 automation as defined by both SAE and NHTSA, the vehicle-following performance that can be achieved under each scenario is representative of the performance that should be expected at higher levels of automation. Implementation of CACC in practice will also require consideration of more than the lowest level of vehicle-following and speed regulation performance. Because CACC requires interactions between adjacent equipped vehicles, strategies are needed such as ad hoc, local, or global coordination to cluster CACC vehicles. Some of the challenges that must be overcome to implement the clustering strategies are discussed as well as strategies for separating CACC clusters as they approach their destinations, as potential traffic improvements from CACC will be negated if the vehicles cannot disperse effectively.

Data collected from the states of Minnesota and Washington on rural two-lane highways are used to build accident models for segments and three-legged and four-legged intersections stop-controlled on the minor legs. The quantity, quality, and variety of data collected, together with the advanced techniques applied in the analysis, make this study of special interest. Variables include traffic, horizontal and vertical alignments, lane and shoulder widths, roadside hazard rating, channelization, and number of driveways. Models are of negative binomial and extended negative binomial form and yield R 2 values from 0.42 to 0.73 and overdispersion parameters from 0.20 to 0.51. A segment model combining both states and including state as a variable, and intersection models derived from Minnesota data, are featured, along with summary statistics, goodness-of-fit measures, and cross-validation between the states. Segment accidents depend significantly on most of the roadway variables collected, while intersection accidents depend primarily on traffic. The study recommends development of adjustment factors for different regions and times and further development of extended negative binomial models.

Significant recent advancements in cement-based, fiber-reinforced composite materials have stretched the bounds of concrete into the realm of ultrahigh-performance concrete (UHPC). The durability of a commercially available UHPC was independently evaluated through six standardized durability tests and the results are reported herein. Regardless of the curing treatment applied, this concrete exhibits significantly enhanced durability properties as compared to normal and high performance concretes. The concrete exhibited minimal damage after being subjected to two times the normal number of ASTM C 666 freeze–thaw cycles. It was innocuous to ASTM C 1260 ASR deterioration, to ASTM C 672 scaling deterioration, and to AASHTO T259 chloride penetration. The ASTM C 1202 Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration test result was negligible if any steam-based curing treatment was applied, and was very low otherwise. Steam-based curing treatments significantly enhanced the ASTM C 944 abrasion resistance. The curing treatment applied to this concrete can affect the durability properties, partly due to steam treatment increasing the degree of hydration of the concrete, improving its microstructure, and decreasing its permeability.

The flexural behavior of an ultrahigh-performance concrete (UHPC) was investigated through the testing and related analysis of a full-scale prestressed I-girder. A 28ksi(193MPa) compressive strength steel fiber reinforced concrete was used to fabricate an 80ft(24.4m) long AASHTO Type II girder containing 26 prestressing strands and no mild steel reinforcement. Intermediate and final behaviors, including cracking, flexural stiffness, and moment capacity, were investigated. Test results are compared to predictions based on standard analytical procedures. A relationship between tensile strain and crack spacing is developed. The uniaxial stress-strain response of UHPC when subjected to flexural stresses in an I-girder is determined and is verified to be representative of both the stress and flexural stiffness behaviors of the girder. A flexural design philosophy for this type of girder is proposed.

The performance of asphalt concrete (AC) mixtures is influenced by its internal structure, which refers to the arrangement of aggregates and their associated air voids. Currently, most of the discussion on the effects of internal structure on AC performance is qualitative. This study proposes computer-automated image analysis procedures to quantify the internal structure of AC. Internal structure is quantified in terms of aggregate orientation, aggregate contacts, and air void distribution. The new procedures are useful tools to describe and compare AC materials produced by different compaction methods and mix designs. The new procedures are used to study the difference in internal structure of AC specimens compacted with the Superpave gyratory compactor (SGC) and the linear kneading compactor (LKC). Specimens compacted with the SGC were found to have aggregates with more preferred orientation and fewer contacts than specimens compacted with the LKC. In addition, SGC specimens were found to have more air voids at the top and bottom, whereas air voids in LKC specimens were found to increase from the top to the bottom.

Congestion is one of the biggest challenges faced by the transportation community; congestion accounted for an estimated $87.2 billion in losses in 2007 alone. Transportation professionals need to go beyond capacity expansion projects and explore novel strategies to mitigate traffic congestion. Automated intersection management is a novel strategy that has the potential to greatly reduce intersection delay and improve safety. Although the implementation of such a system is contingent on the development of automated vehicles, competitions such as the Grand Challenge and Urban Challenge of the Defense Advanced Research Projects Agency have shown that this technology is feasible and will be available. Therefore, the development of the infrastructure and associated control methods required to exploit fully the benefits of such technology at the system level is critical. This research explores one such innovative strategy, an automated intersection control protocol based on a first-come, first-served (FCFS) reservation system. The FCFS reservation system was shown to reduce intersection delay significantly by exploiting the features of autonomous vehicles. Microscopic simulation experimental results showed that the FCFS reservation system significantly outperformed a traditional traffic signal in reducing delay.

The results of research involving a well-designed before-and-after evaluation of the safety effects of providing left- and right-turn lanes for at-grade intersections are presented. Geometric design, traffic control, traffic volume, and traffic accident data were gathered for a total of 280 improved intersections as well as 300 similar intersections that were not improved during the study period. The types of improvement projects evaluated included installation of added left-turn lanes, added right-turn lanes, and extension of the length of existing left- or right-turn lanes. An observational before-and-after evaluation of these projects was performed by using several alternative evaluation approaches. Three contrasting approaches to before-and-after evaluation were used: the yoked comparison or matched-pair approach, the comparison group approach, and the empirical Bayes approach. The research not only evaluated the safety effectiveness of left- and right-turn lane improvements but also compared the performances of these three alternative approaches in making such evaluations. The research developed quantitative safety effectiveness measures for installation design improvements involving added left-turn lanes and added right-turn lanes. The research concluded that the empirical Bayes method provides the most accurate and reliable results. Further use of this method is recommended.

The past few years have seen a rapid evolution in the sophistication of traffic microsimulation models and a consequent major expansion of their use in transportation engineering and planning practice. Researchers and practitioners have employed an extensive array of approaches to calibrate these models and have selected a wide range of parameters to calibrate and a broad range of acceptance criteria. A methodical, top-down approach to model calibration is outlined; it focuses the initial effort on a few key parameters that have the greatest impact on model performance and then proceeds to less critical parameters to finalize the calibration. A three-step calibration/validation process is recommended. First, the model is calibrated for capacity at the key bottlenecks in the system (the capacity calibration step). Second, the model is calibrated for traffic flows at nonbottleneck locations in the system (the route choice calibration step). Finally, the overall model performance is calibrated against field-measured system performance measures such as travel time and delay (the system performance calibration step). This three-step process is illustrated in an example application for a freeway/arterial corridor.

The Superpave specification parameter |G*|/sin δ for high temperature performance grading of paving asphalts has not been found to be adequate in rating various binders, especially some polymer-modified ones, for their rutting resistance. This has led researchers to seek other possible parameters that may better relate to rutting resistance and also to search for ways to improve the existing parameter |G*|/sin δ so that it is more sensitive to pavement performance. Some researchers have suggested the repeated creep and recovery test, while others have used a semiempirical approach as a means to refine the existing Superpave specification parameter. The present work revisits the proposed refinements of the Superpave specification parameter and shows that the semiempirical approach involving curve-fitting of experimental data is not necessary if the derivations are based on fundamental concepts. The final equations obtained through a theoretical development are verified using part of the same experimental data that were used by the earlier researchers.

Cooperative Adaptive Cruise Control (CACC) allows vehicles to exchange real-time operational information wirelessly, enabling vehicles to travel in strings with shorter than normal time gaps between adjacent vehicles and ultimately increasing the freeway capacity. This study is intended to investigate the impact of CACC vehicle string operation on the capacity of multilane freeway merge bottlenecks, commonly found at on-ramp merging areas on urban freeways. Simulation experiments were conducted using CACC car-following models derived from field test data, together with lane-changing models of CACC vehicles and manually driven vehicles, as well as a maximum CACC string length and inter-string time gap constraint. Simulation results reveal that the freeway capacity increases quadratically as the CACC market penetration increases, with a maximum value of 3080 veh/hr/lane at 100% market penetration. The disturbance from the on-ramp traffic causes the merge bottleneck and can reduce the freeway capacity by up to 13% but the bottleneck capacity still increases in a quadratic pattern as CACC market penetration becomes larger. The findings suggest that there is a need to implement advanced merging assistance systems with CACC at merge bottlenecks for achieving the capacity improvement comparable with the observations at homogeneous freeway segments.

Air void distribution has considerable influence on the mechanical properties of asphalt mixtures. Several factors such as the compaction effort, method of compaction, aggregate gradation, and aggregate shape control the air void distribution. An X-ray computed tomography (CT) system along with image analysis techniques are used in this study for non-destructive characterization of air void distribution in gyratory specimens prepared using different gradations and compaction efforts. The air void distributions in gyratory specimens are quantified using parameters that describe the change in percent and volume of air voids along the horizontal and vertical directions. Air voids are shown to be more concentrated in the top and bottom regions that are in contact with the base plates, as well as in the outer region that is in contact with the mold. The non-uniformity of the distribution increases with an increase in compaction effort. The difference in aggregate gradations used in this study is shown to have just a slight influence on the air void distribution.

Network screening is the first step in the site safety improvement process. The product of network screening is a list of sites that are ranked by priority for the conduct of detailed engineering studies. In turn, cost-effective projects are formulated from the studies. With the purpose of laying the foundation for improved network screening, the role of network screening is clarified, and how project cost and safety benefit can be anticipated at the time of screening is examined. The strengths and weaknesses of alternative assumptions on which the anticipation of safety benefit can be based are discussed. A way to guard against misallocation of resources due to the randomness of accident counts is suggested, and a method for finding peak sites within road sections is proposed.

A bridge with lead-rubber seismic isolation bearings was field-tested to evaluate the assumptions and uncertainties in the design and construction. A numerical model was established based on satisfactory predictions of the dynamic characteristics under ambient vibration, free vibration tests, and the seismic performance under the 1022 Gia-Yi earthquake. Parametric studies of this model, under simulated near-fault ground motions, were carried out to investigate the near-fault effect. Two types of velocity pulse that characterize the near-fault effect were investigated. Results revealed amplified seismic response when the pulse period was close to the effective period. The response amplification by a longer period pulse was not consistent for the two pulse types. Additional observations included the ratio of dissipated energy by the lead-rubber bearings to the total input energy, which could be influenced by the near-fault effect. Variation of the near-fault effect due to a change of structural parameters was studied to provide information on possible mitigation strategy.

ACS-Lite is being developed by FHWA to be a cost-effective solution for applying adaptive control system (ACS) technology to current, state-of-the-practice closed-loop traffic signal control systems. This effort is intended to make ACS technology accessible to many jurisdictions without the upgrade and maintenance costs required to implement ACS systems that provide optimized signal timings on a second-by-second basis. The ACS-Lite system includes three major algorithmic components: a time-of-day (TOD) tuner, a run-time refiner, and a transition manager. The TOD tuner maintains plan parameters (cycle, splits, and offsets) as the long-term traffic conditions change. The run-time refiner modifies the cycle, splits, and offsets of the plan that is currently running based on observation of traffic conditions that are outside the normal bounds of conditions this plan is designed to handle. The run-time refiner also determines the best time to transition from the current plan to the next plan in the schedule, or, like a traffic-responsive system, it might transition to a plan that is not scheduled next in the sequence. The transition manager selects from the transition methods built in to the local controllers to balance the time spent out of coordination with the delay and congestion that is potentially caused by getting back into step as quickly as possible. These components of the ACS-Lite algorithm architecture are described and the similarities and differences of ACS-Lite with state-of-the-art and state-of-the-practice adaptive control algorithms are discussed. Closed-loop control system characteristics are summarized to give the context in which ACS-Lite is intended to operate.

A speed-profile model for estimating 85th percentile speeds along horizontal alignments of rural two-lane highways in the United States is documented. The model is an evaluation tool to check for speed consistency violations on alignments with design speeds less than 100 km/h (62.1 mph). The model was calibrated by using speed and geometry data collected for 138 horizontal curves and 78 approach tangents on 29 rural highways in 5 states. A preliminary evaluation suggests that the model provides reasonable estimates of the reductions in 85th percentile speeds from an approach tangent to a horizontal curve.

Unreinforced concrete or concrete reinforced with noncorrosive reinforcement could potentially be mixed with seawater in locations where potable water is scarce. A fundamental understanding of the properties of concrete mixed with seawater is therefore essential. This paper analyzes the hydration kinetics, hydrate phases, pore solution, and porosity of cementitious pastes made with seawater and compares these results with the corresponding ones from pastes made with deionized water. Pastes were prepared with cement and with a 20% mass replacement of the cement with fly ash. Isothermal calorimetry (to study hydration kinetics), thermogravimetric analysis (to study the hydrated phase assemblage), X-ray fluorescence (to determine pore solution composition and electrical resistivity), and dynamic vapor sorption (to determine the pore size distribution) were performed on the paste samples. Seawater accelerates hydration kinetics at an early age; however, this effect is negligible at later ages. Friedel’s salt formation in systems with seawater at later ages is negligible [0.4% (by mass of paste) at 91 days]. The primary difference between the hydrated phases of pastes made with seawater and those made with deionized water appears to be the absorption of chloride in the calcium silicate hydrate. The pore solution in pastes made with seawater has higher sodium, chloride, and hydroxide ion concentrations. The concentrations of sodium, potassium, and hydroxide ions in pore solutions are lower in pastes with fly ash compared to pastes without fly ash. Pastes with seawater show a lower electrical resistivity than pastes with deionized water due to the higher ionic concentrations. Paste with seawater has a slightly finer pore structure compared to paste with deionized water.

This study uses the discrete element method (DEM) to simulate the dynamic mechanical behaviour of asphalt mastics. Asphalt mastics are defined as dispersions of aggregate fillers within a medium of asphalt binder. The fillers refer to the fraction of mineral aggregate passing the 200-mesh sieve, (i.e. smaller than 75 μm). Mastic measurements obtained using the dynamic shear rheometer were compared to DEM predictions and available micromechanics-based models. Three asphalt binders (ABD-1, AAM-1 and ABM-1) and four mineral fillers (glacial gravel, granite, limestone and greywacke) were used to prepare the mastics in this study. The stiffening effect of the mineral fillers was investigated at different filler volume fractions. The DEM results captured the stiffening behaviour of asphalt mastics as a function of the volumetric concentration of mineral fillers. The DEM results exhibited a high rate of stiffening that is typically observed in experimental measurements of mastics at relatively low volume concentrations of fillers. Compared to the DEM results, the micromechanics-based models were not sensitive to the dynamic shear modulus of the asphalt binder, and they underestimated the stiffening effect of the mineral fillers.

In this paper, an approach to computational flutter analysis is presented, which permits the determination of the critical wind velocity that initiates damping‐ and stiffness‐driven flutter of cable‐suspended bridges. Dynamic response may be coupled elastically and/or aerodynamically, with the developed algorithm, the so‐called pK‐F method determining the preflutter and postflutter responses by solution to the modal equations of motion. The entire method is presented in matrix form, so as to be easily implemented into finite element systems. As an example, the pK‐F method is applied to the Luling cable‐stayed bridge, which was experimentally tested for flutter in the wind tunnel. Also, examples are provided for flutter determination with different deck sections. The pK‐F method has proved reliable in its methodology and efficient in its use. The generality of the method permits more flutter scenarios to be examined, including construction stages and different flutter derivatives for different portions of the bridge structure.