Showing papers in "Transportation Research Record in 1991"

Alinea: a local feedback control law for on-ramp metering

Abstract: ALINEA, a new local traffic-responsive strategy for ramp metering, is presented. The new control strategy is based on a feedback structure and is derived by use of classical automatic control methods. ALINEA is a simple, robust, flexible, and effective local strategy for ramp metering. Real-life results from application of the new strategy to a single on-ramp of the Boulevard Peripherique in Paris are provided.

Freeway capacity drop and the definition of capacity

Abstract: Two aspects of the definition of freeway capacity are considered here. The first is whether there is a reduction of maximum flow rates when a queue forms. There appears to be roughly a 6% reduction in maximum flow rates after the onset of congestion, but not of the type discussed in the current Highway Capacity Manual (HCM). An indirect issue arises during this analysis pertaining to the question of where capacity can properly be measured. The answer, not surprisingly, is that it can only be measured in a bottleneck, and not in a queue. However, the HCM discussion identifies the potential capacity drop on the basis of operations in a queue. The analysis contained here explains why that is inappropriate. The second aspect considered is the distribution over time of maximum flows at a single location. The distribution approximates a normal one reasonably well, with a mean of 6,071 and a standard deviation of 262 vehicles per hour. The unanswered question from the analysis is what portion, or percentile, of the distribution is appropriate to use to meet the HCM definition of capacity, which calls for a value that can "reasonably be expected" to be achieved.

Is Jobs-Housing Balance a Transportation Issue?

Abstract: Jobs-housing balance has become a major planning and public policy issue. Despite its popularity and apparent acceptance among public policy makers as a solution for traffic congestion and air pollution problems, there is little consensus on what jobs-housing balance means and little evidence that a jobs-housing balance policy would have any significant effect on these problems. The jobs-housing balance policy is premised on the idea that job and housing location choices are closely linked, and that policy intervention is required to achieve a balance of housing and jobs. Existing evidence suggests that the relationship between where people choose to live and work is complex, and may have little to do with job access considerations. Further, patterns of urban growth and travel indicate that balancing occurs as part of the urban development process. It is concluded that jobs-housing balance is not an effective solution for traffic congestion and air pollution concerns. Rather, these problems are better addressed in a more direct way.

Distribution of wheel loads on highway bridges

Abstract: This report presents the findings of the research performed on distribution of wheel loads on highway bridges under funding from the National Cooperative Highway Research Program (NCHRP). This study was performed in two phases; the first phase (NCHRP 12-26) concentrating on beam and slab, and box girder bridges; and the second phase (NCHRP 12-26/1) concentrating on slab, multi-box beam, and spread box beam bridges. For each bridge type three levels of analysis are considered. The most accurate level, Level-three, involves detailed modeling of the bridge deck. Level-two includes graphical methods, nomographs, influence surfaces; or simplified computer programs developed to apply such methods. Level-one methods include simple formulae to predict lateral load distribution to various girders using a wheel load distribution factor. This research has resulted in a number of more accurate formulae for wheel load distribution, and recommendations for use of computer programs to achieve more accurate results. These recommendations focus on use of plane grid analysis as well as detailed finite element analysis. Also, as a result of this research, a draft specification is prepared for determination of wheel load distribution factors. This draft specification is recommended to replace the current AASHTO specification.

Two-capacity phenomenon at freeway bottlenecks: a basis for ramp metering?

Abstract: The issue of whether ramp metering can increase the capacity of freeway bottlenecks by prevention or delay of flow breakdown on the freeway main line is considered. A summary of the results of four case studies of metered bottlenecks in San Diego is presented. The hypothesis that flow decreases when it breaks down is confirmed, provided the hypothesis applies to individual lanes. Flow decreases ranging from 10% to less than 1% were observed in the left lane at the various study sites. When averaged across all lanes, flow decreased by about 3% at one site; there was no significant change at the other sites. It is concluded that this phenomenon is unlikely to provide a basis for metering at more than a few locations, because decreases in flow across all lanes are very small, sometimes nonexistent. Even where there are decreases in total flow, there is risk that metering will be counterproductive if it is too restrictive or begins too early.

Planning for a comprehensive bridge safety assurance program

Abstract: With the increasing number of structurally deficient and functionally obsolete bridges and recent catastrophic bridge failures in the United States, the need to strengthen bridge safety has become pronounced. This paper presents considerations in planning a comprehensive bridge safety assurance program, including identification of potential causes and modes of bridge failures based on review of failures, imminent failures and actual closures. As these causes and modes of failure are identified, they are prioritized in terms of their potential impact on the structures for which an agency is responsible. The paper discusses steps to develop a rating system that categorizes and ranks bridges by their relative vulnerability to the various failure modes. Further, it presents a method of preparing initial screening lists based on characteristics of the site and structure, verifying such lists, and then categorizing them according to appropriate action-needed categories. Preparation of short and long-term strategies to reduce or eliminate vulnerability of bridge failures is also discussed.

Using an interactive route-choice simulator to investigate drivers' compliance with route guidance advice

Abstract: Possible sources of data on drivers' reactions to route guidance are discussed. Field evidence is sketchy and appears likely to remain so for some time. It is argued that an interactive route-choice simulator might provide acceptable substitute data. The design and development of such a simulator, interactive guidance on routes (IGOR), is described; users make a series of journeys through test networks by indicating their desired exit from each junction they reach. At each junction IGOR displays a plan giving traffic conditions, and so on. For some journeys the user has access to a map of the network, guidance advice, or both. The advice system replicates in-vehicle systems, which advise the driver what exit to take at each junction in order to minimize journey time in the current traffic conditions. To ascertain the effect of variations in the quality of guidance on user response, a "wrong" exit is sometimes recommended. The use of IGOR to collect data under the Dedicated Road Infrastructure for Vehicle Safety in Europe initiative is described and important results are presented. It is observed that acceptance of an item of advice depends on its (objective) quality, the quality of previously received advice, the drivers' knowledge of the network, and on the extent to which advice is corroborated by other evidence. Compliance with advice is a function of its credibility and this in turn depends on past experience, local conditions, and psychological factors. The value of IGOR and its results are discussed. Plans for use and further analysis of the IGOR data are outlined together with some options for further development of the concept.

Safety effects of geometric improvements on horizontal curves

Abstract: The purpose was to (a) determine the horizontal curve features that affect accident experience on two-lane rural roads, (b) determine which types of geometric improvements on curves will affect accident experience, and (c) develop accident reduction factors based on these findings. Very little of this information has been available to highway safety engineers and designers. The results were based on an analysis of 10,900 horizontal curves in Washington State with corresponding accident, geometric, traffic, and roadway data variables. Statistical modeling revealed significantly higher curve accidents for sharper curves, narrower curve width, lack of spiral transitions, and increased superelevation deficiency. All else being equal, higher traffic volumes and longer curves were also associated with significantly higher curve accidents. Ranges of accident reductions for horizontal curves improvements were determined for flattening curves, widening lanes, widening paved shoulders, adding unpaved shoulders, adding a spiral transition, and improving superelevation. From the study findings, a variety of improvements were recommended for horizontal curves with moderate and sharp curvature, and upgrading specific roadside improvements. Expected costs should be compared with estimated accident reductions to determine whether geometric improvements are warranted.

Effect of lime on volume change and compressibility of expansive clays

Abstract: Heave and settlement of clayey soils pose a difficult problem to civil engineers. Several methods are usually suggested to control this problem. The most common method is the addition of stabilizing agents, such as lime. An evaluation of the soil-lime system for two soils typical of the highly expansive soils existing in Irbid city in northern Jordan is presented. The lime was added to the soils at 0 to 9 or 12%. The soil-lime specimens were cured for 1 hr, 7 days, and 28 days, after which they were subjected to laboratory tests. The properties obtained were the grain size distribution, consistency limits, chemical composition, swell potential, swell pressure, compression and rebound indices, rate of swell and consolidation, immediate settlement, and primary consolidation as percent of total settlement. Generally, lime is found to be most effective in stabilizing heave and settlement of expansive clays.

The two-capacity phenomenon: some theoretical issues

Abstract: Theoretical issues related to two phenomena observed in a study of four freeway bottlenecks in San Diego are addressed. It was observed that flow immediately downstream of the bottlenecks decreased by a small amount when it broke down and that flow breakdown appeared to be triggered by speed instability. Most of the flow decrease could be attributed to the increase in vehicle passage time that occurs when speeds decrease, and most of the San Diego data are compatible with the linear car-following model of Chandler et al. as extended by Bexelius, although a number of questions about the validity and applicability of this model remain.

Age differences in motion perception and specific traffic maneuver problems

Abstract: Studies of age differences in motion perception abilities and accident involvement patterns are reviewed to predict the exaggerated difficulty for older drivers with specific traffic maneuvers and the expected ordering of older-driver accident involvement rates by type of maneuver. A rigorous analysis of police-reported accidents in Michigan and Pennsylvania used induced exposure methods to demonstrate the highest older-driver accident involvement rate for turning left against oncoming traffic; the next highest older-driver accident rate occurred when drivers were crossing or turning into a traffic stream, although the extent of overinvolvement was not as high as the first situation; and the lowest relative involvement rates for maneuvers in which an age-related motion perception deficit may be at issue were in situations in which vehicle headways are critical, such as overtaking. Mitigating factors such as older drivers' slower driving that may compensate for or minimize some of the problems found in laboratory tests are noted. Overall, the present review and analysis supports an interpretation that age differences in motion perception in critical traffic situations is an important factor in older drivers' overinvolvement in particular accident types.

Evaluation of optimized policies for adaptive control strategy

Abstract: Optimization Policies for Adaptive Control (OPAC) is an on-line control algorithm designed to optimize the performance of individual traffic signals. It is a building block for demand-responsive control of a distributed signal system. OPAC-RT is a traffic signal control system that implements the OPAC strategy in real time. The system uses traffic data collected from detectors located well upstream (400 to 600 ft) of the stop bar on all approaches to an intersection. Signal timings are dynamically optimized in a demand-responsive manner using a rolling horizon scheme. Results of the first implementation and field testing of the on-line OPAC strategy at individual intersections indicate that OPAC performs better than well-timed actuated signals, particularly at greater demand levels. The payback period of the incremental costs of installing the current version of OPAC is estimated to be less than 1 year. Further enhancements of the OPAC system operation are likely to significantly increase its effectiveness.

Analysis of pedestrian movements in bangkok

Abstract: Walking rates of pedestrians in Bangkok, Thailand, are analyzed. Data were collected using a photographic technique with the aid of a video camera on selected walkways such as sidewalks, stairways, and crosswalks. Walking rates on the pedestrian facilities in Bangkok were determined and compared with those of Western standards as well as with findings obtained in other Asian countries. The findings of the study confirmed that Asian pedestrians walk slower compared with their Western counterparts, so local design standards are needed for pedestrian facilities in Asian countries.

Use of speed limiters in cars for increased safety and a better environment

Abstract: A major traffic safety problem exists in Europe because many road users do not comply with speed limits. Therefore, researchers at the Department of Traffic Planning and Engineering at Lund Institute of Technology in Sweden have developed a method for controlling speed at the source, that is, in the vehicle. This method involves use of a speed limiter (SL). At every change of existing speed limit, a sender emits impulses. Each vehicle is equipped with a receiver able to understand these impulses, automatically limiting the vehicle's maximum speed to the speed limit in question. This concept sounds utopic, and there is not the slightest chance of introducing an SL into road traffic unless the advantages, from safety and environmental viewpoints, clearly outweigh the possible disadvantages. Hypotheses about the SL's major impact on traffic were developed through a literature study, roundtable discussions, and self-observation studies. In addition to objective safety, these hypotheses concern security, travel time, road net capacity, energy consumption, air pollution, and noise. External test drivers are currently being chosen for behavior observations and interviews. Their attitudes concerning several safety aspects will be measured, then groups of drivers will use the SL for different lengths of time. After driving, they will be interviewed again about their attitude toward the SL and its practical use. Along with the field experiments, estimates will be made on aggregated levels. The results of these studies will be used in planning and designing a large-scale experiment that will be held as an option, unless there is a surprising change of status quo in the official attitude concerning an SL system.

Multiple user class assignment model for route guidance

Abstract: The application of the concept of multiple user class equilibrium assignment to the modeling of route guidance systems is the concern of this paper. In particular, its role in modeling guided and unguided drivers is discussed, as well as its abilty to lead to guidance strategies that are effective even with a large proportion of drivers equipped. A review of previous route guidance model work is given. A multiple user class model of route guidance is then proposed and the properties of such a model are discussed. Finally, a presentation of simulation results obtained from such a model, for two real-life networks and for a number of route guidance scenarios, is given.

Improved data screening techniques for freeway traffic management systems

Abstract: Proper functioning of computer-based freeway traffic control algorithms depends on good data being received from roadway detectors. At present, only elementary screening procedures for these data are in use on most freeway traffic management systems. There has recently been some work to improve such procedures. That work has been extended, and several new screening approaches are proposed. The first approach develops a theoretical upper bound for part of the flow-occupancy data and is useful for single-detector systems. The other approaches have been developed for paired-loop systems and involve comparisons of loop data as well as the development of boundaries defining acceptable combinations of speed, flow, and occupancy data.

Simulation of crowd behavior and movement: fundamental relations and application

Abstract: A model of crowd behavior and movement in the Jamarat system, which is a critical bottleneck during the Hajj, the Muslims' annual pilgrimage to Makkah, Saudi Arabia, is described. The model consists of a set of partial differential equations that are solved numerically following a discretization of time and space. Mathematical relations are defined for three fundamental processes: (a) radial movement, (b) lateral movement, and (c) stoning process. These relations are developed and calibrated using actual measurements taken at the site. As a result, a bidirectional speed-concentration model is developed to describe radial movement, revealing that the impedance to movement from facility users going in the same direction is about twice that from those going in the opposite direction. The overall model is applied to the evaluation of possible design and control strategies aimed at improving the efficiency and throughput of the system.

Comparison of central business district pedestrian characteristics in canada and sri lanka

Abstract: Characteristics of pedestrians in the central business district of Colombo, Sri Lanka, are described. The characteristics measured included walking speed, flow, and density for sidewalks of varying widths. Comparisons are made with results from Calgary, Canada, and from other Asian studies. The findings indicate that Asian speeds for all groups are significantly lower than those observed in Calgary, Canada. Implications of the findings with respect to the technological transfer of pedestrian planning techniques between the two countries are also discussed. The differences in pedestrian characteristics are caused not only by differences in speed, flow, and density relationships, but also by cultural differences. Problems common to both countries are lack of pedestrian data and difficulties involved in collecting pedestrian data. Suggestions concerning low-cost methods of updating pedestrian count data are provided.

Analysis of crack propagation in asphalt concrete using a cohesive crack model

Abstract: A cohesive crack model, which is similar to the Dugdale-Barenblatt model, was proposed to simulate the progressive crack development in asphalt concrete. Tensile strength, fracture energy, and the stress-separation relationship are the basic material properties associated with this model. To evaluate the material properties, indirect tensile tests and three-point bend tests were performed. From these experimental results, the effects of temperature on Young's modulus, the fracture energy, and the indirect tensile strength were evaluated. To determine the stress-separation relationship, a numerical simulation (or curve-fitting method) was used. Using the material properties obtained from the experimental study, temperature effects on different fracture parameters (i.e., critical stress intensity factor and critical J-integral) were studied. The theoretical predictions were found to be in good agreement with the available experimental results. This finding also indicates the potential applications of the proposed model in evaluating the performance of asphalt concrete pavements.

Forecasting intercity rail ridership using revealed preference and stated preference data

Abstract: A methodology for incorporating revealed preference (RP) and stated preference (SP) data in discrete choice models is presented. The methodology is applied to intercity travel mode choice analysis. New mode shares for each origin-destination pair resulting from changes in service levels are predicted. The combined estimation technique with RP and SP data is developed to promote advantages of the two complementary data sources. The empirical study of intercity travel demand demonstrates the practicality of the methodology by accurately reproducing observed aggregate data and by applying a flexible operational prediction method.

Gaps accepted at stop-controlled intersections

Abstract: Gap-acceptance data are used to determine intersection sight distance, capacity, queue length, and delay at unsignalized intersections. They have also been used to determine the need for a traffic signal, the capacity of a left-turn lane, and warrants for left-turn signal phasing and storage lanes. A field study was performed to determine the gap-acceptance values of truck and passenger car drivers at six intersections. Each intersection was formed by two 2-lane roads; the minor road was controlled by a stop sign. The data obtained in the field were evaluated by three methods: Greenshield, Raff, and logit. The findings from the field studies were summarized into generalized values. Passenger car drivers had a 50% probability of accepting a gap of 6.5 sec for both left and right turns and an 85% probability of accepting a gap of 8.25 sec at a moderate- to high-volume intersection. A 10.5-sec gap represented the 85% probability of accepting a gap at an intersection where accepted gaps were influenced by low volume and the intersection's geometry. Truck drivers' 50% probability of accepting a gap was 8.5 sec. At a high-volume location, 85% of the truck drivers accepted a 10-sec gap; at a low-volume location, 15.0 sec was the accepted gap value.

Effect of aggregate gradation variation on asphalt concrete mix properties

Abstract: Six asphalt concrete mixes were tested to investigate the effects of variation in the aggregate gradation on mix properties. The asphalt contents of the mixes were maintained at the job mix design contents. The gradation variations were representative of typical construction extremes. Five gradations were tested from each mix: (a) the job mix formula (JMF) gradation, (b) a fine gradation, (c) a coarse gradation, (d) a coarse-fine gradation, and (e) a fine-coarse gradation. The fine and coarse gradations deviated from the JMF gradation by the maximum amount to the fine or coarse side. The fine-coarse and coarse-fine gradations crossed over the JMF gradation curve from the maximum fine (or coarse) amount on the largest size fraction to the maximum coarse (or fine) amount on the smallest size fraction. Properties investigated were creep stiffness, split tensile strength, resilient modulus, Marshall stability, Marshall flow, air voids, and voids in mineral aggregate. Analysis of the data revealed that the fine-coarse and coarse-fine gradation variations had the greatest impact on mix properties but that none of the variations had a significant effect on resilient modulus. The data also showed that within the range normally encountered, air void content had a greater impact on split tensile strength than did gradation variation.

Convergent algorithm for dynamic traffic assignment

Abstract: A link flow formulation and a convergent solution algorithm for the dynamic user equilibrium (DUE) traffic assignment problem for road networks with multiple trip origins and destinations are presented. The link flow formulation does not implicitly assume complete enumeration of all origin-destination paths as does the equivalent path flow formulation. DUE is a temporal generalization of the static user equilibrium (SUE) assignment problem with additional constraints to ensure temporally continuous paths of flow. Whereas SUE can be solved by methods of linear combinations, these methods can create temporally discontinuous flows if applied to DUE. This convergent dynamic algorithm (CDA) uses the Frank-Wolfe method of linear combinations to find successive solutions to DUE while holding node time intervals fixed from each origin. In DUE, the full assignment period of several hours is discretized into shorter time intervals of 10 to 15 min each, for which trip departure matrices are assumed to be known. The performance of CDA is compared with that of a heuristic solution procedure called DTA. CDA can be applied to solving DUE on large networks, and the examples presented show that CDA consistently converges to solutions that closely satisfy the DUE optimality conditions. With computational advances such as parallel computing, CDA can be run in near real-time on large-scale networks and used with in-vehicle route advisory systems for traffic management during evacuations and special events.

Comparative assessment of origin-based and en route real-time information under alternative user behavior rules

Abstract: The effects of real-time traffic information, supplied at the origin of the trip or along the way (en route), on the system's performance under alternative behavioral rules governing path selection in the network are examined in this paper. Simulation experiments are performed to investigate the effect on overall system performance as well as the incidence of benefits (costs) across user information groups of four experimental factors: (a) behavioral rules, governing users' response to real-time information, (b) sources of information, consisting of point-of-departure or in-vehicle (or both), (c) prevailing "initial conditions" in the system, and (d) market penetration (i.e., the fraction of users with access to real-time information in the network). The results of these simulation experiments provide insights into the effectiveness of real-time advanced driver information systems on systemwide performance and on its critical determinants. The results confirm a priori expectations that the existence of benefits as well as the relative effectiveness of origin-based versus en route information is highly dependent on the initial conditions prevailing in the system as well as the behavioral rules governing path selection. Extreme behavior by users (with frequent switching in myopic response to any gain, no matter how small) could lead to severe worsening of traffic conditions under real-time information from either source. On the other hand, switching according to a boundedly rational model incorporating a threshold improvement in trip time is more likely to lead to meaningful systemwide benefits.

Procedure for validation of microscopic traffic flow simulation models

Abstract: Model verification and validation are two important tasks in developing a traffic simulation model. Traffic simulation models have unique characteristics because of the interaction among the drivers, vehicles, and roadway. The effects of the interaction on traffic flow should be considered in verification and validation of the models. If these two tasks are not properly performed, a traffic simulation model may not provide accurate results. A procedure for verification and validation of microscopic traffic simulation models is developed, and its application to a car-following simulation model, CARSIM, is demonstrated. The validation part of the procedure is emphasized. The validation efforts are performed at the microscopic and macroscopic levels. For validation at the microscopic level, the speed change patterns and trajectory plots obtained from simulation models are compared with those from field data. For validation at the macroscopic level, the average speed, density, and volume for simulated platoons are compared with those of field data. Also, variation of these parameters when the platoons go through a disturbance and interrelationships between these variables computed from the simulation models and the field data are examined. Regression analysis and analysis of variance of the simulation results versus the field data are discussed. The procedure may be considered as a step toward development of a comprehensive systematic approach for verification and validation of traffic simulation models.

Epoxy-resin-based chemical stabilization of a fine, poorly graded soil system

Abstract: Results are described of a research effort on the epoxy-based treatment of fine, poorly graded soil found at some localized low-duty airport sites and in the north slopes of Alaska. Statistical models are developed for the stabilization of clay-silt pavement systems at low-duty airports. A nontraditional method of soil stabilization that improves the subgrade strength properties of poorly graded clay-silt was identified. This soil system is considered one of the most difficult soil types to stabilize, in part because of its poor particle size distribution. Among several organic additives tested, the two-part epoxy system--bisphenol A/epichlorohydrin resin plus a polyamide hardener--gave the best result as measured by the dry California bearing ratio (CBR) test. The choice of the dry CBR test performed to ASTM specification was motivated by a need to capture optimum moisture content as an experimental variable. Within the limits of the laboratory test conditions, the statistical regression models developed support the hypothesis that the marginal increase in CBR values caused by a 1% increase in epoxy resin application is 11.1 and the marginal degradation of CBR caused by a 1% increase in moisture level in -5.6. Also, only additive level, moisture content, and temperature are significant variables influencing soil strength.

Proposed analytical technique for estimating capacity and level of service of major freeway weaving sections

Abstract: Weaving typically occurs where merging traffic movements cross over diverging movements. The prevalence of weaving areas on freeways warrants the need for analytical techniques that can reliably analyze or design these critical freeway components. However, previous research at the University of California suggested that existing analytical procedures may not predict weaving operation with a sufficient degree of reliability. Consequently, a more reliable procedure for evaluating weaving performance was developed. Unlike most existing procedures, the proposed technique evaluates traffic flow behavior in individual lanes of the weaving section. The procedure is applicable to major weaving areas, a subset of all weaving configurations. The proposed procedure predicts vehicle flow rates in critical regions within the weaving section as a function of prevailing traffic flow and geometric conditions. Predicted flows are then used to assess the capacity sufficiency and level of service of a subject weaving area. The model itself was developed using large amounts of empirical and simulation data, collected from a number of sites throughout California. Some of the more interesting traffic flow characteristics empirically observed on a single weaving site are highlighted. Moreover, the basic format of the proposed procedure is presented.

Laboratory tests for predicting coarse aggregate performance in ontario

Abstract: Coarse aggregates used in granular base and asphaltic and portland cement concrete must be sound and capable of withstanding the prevailing environment. In Canada, aggregates must be frost resistant. Durability of coarse aggregate is normally evaluated in the sulfate soundness test and water absorption tests, and by measuring resistance to impact in the Los Angeles abrasion and impact test. These tests suffer from some disadvantages: poor precision and inadequate correlation with field performance. The Ministry of Transportation has been developing and evaluating new test procedures for a number of years. Some of the major findings are summarized. The likely performance of aggregates in granular base is best measured by the micro-Deval test and water absorption. The physical quality of portland cement concrete aggregates is best measured by the micro-Deval test, water absorption, and unconfined freezing and thawing. The quality of asphaltic concrete aggregates is best measured by the micro-Deval test, polished-stone value test, and unconfined freezing and thawing test. Petrographic examination is also an essential tool in the evaluation of aggregate quality.

Asphalt hardening in extraction solvents

Abstract: Hardening occurs in asphalts during extraction and recovery, posing a serious problem for research into the properties of hot-mix and road-aged asphalts. Hardening appears to occur in all solvents and, to varying degrees, in all asphalts. The effects of light, oxygen, and temperature on this phenomenon were investigated. All are significant, and the data indicate that reflux methods of extraction should be avoided when the properties of the extracted material are to be investigated. The mechanism of the hardening is still unclear. Infrared spectra show definite chemical changes, but these are quite asphalt-dependent and do not relate clearly to changes in viscosity.

Capacity and delay characteristics of two-way stop-controlled intersections

Abstract: Three objectives are sought: (a) to present a data base for two-way-stop-controlled (TWSC) intersections that can be used to investigate the factors that influence delay and capacity, (b) to identify some of the factors that affect delay and capacity at a TWSC intersection, and (c) to develop a set of preliminary models to estimate delay and capacity. Traffic flow, delay, and geometric data were collected at nine TWSC intersection sites in the Pacific Northwest encompassing a total of 13 hr of intersection operation. A total of 970 minor-street (subject approach) vehicles were observed and nearly 2,000 accepted and rejected gaps were identified and recorded. Each site has several common characteristics: four approach legs, single lanes on aeach approach, and a 25-mph speed limit on the major street. Several factors were identified that may influence delay and capacity at a TWSC intersection. The time waiting in queue (queue time) is affected by the traffic flow rate on the subject and opposing approaches, whereas the time spent waiting at the stop line (service time) is affected by the traffic flow rate on the conflicting approaches. The capacity of the subject approach is also affected by the flow rate on the conflicting approaches. The size of the accepted gap is affected by the length of time that a vehicle has been delayed, the flow rate on the conflicting approaches, and the directional movement of the subject vehicle.