A specialized equilibrium assignment algorithm for air quality modeling

A multiple user class equilibrium assignment algorithm is formulated to determine vehicle trips and the vehicle miles of travel (VMT) in various operating modes on highway links. A heuristic solution algorithm based on the Frank–Wolfe decomposition of the equilibrium assignment problem is presented. The treatment of intrazonal trips, which are very important for emission studies is also discussed. The solution algorithm is implemented in a traffic assignment program for emission studies, referred to as TAPES. The use of the algorithm is demonstrated through a TAPES model case study on a Charlotte, NC network database for 1990 AM peak period. The operating mode mix of VMT in cold transient, hot transient and hot stabilized modes, also known as the mix of cold-starts, hot-starts and stabilized mode trips, is derived on a link by link basis. The results are aggregated by facility type and the location of link segments. It is observed that the operating mode fractions in transient and stabilized modes could vary widely across different facility types geographic locations. The aggregated operating mode fractions derived from the assignment analysis indicates that a lesser proportion of VMT operates in cold and hot transient modes when compared to the operating mode mix derived from the Federal Test Procedure (FTP).     

Frequency-based transit assignment considering seat capacities

This paper proposes a frequency-based assignment model that considers travellers probability of finding a seat in their perception of route cost and hence also their route choice. The model introduces a “fail-to-sit” probability at boarding points with travel costs based on the likelihood of travelling seated or standing. Priority rules are considered; in particular it is assumed that standing on-board passengers will occupy any available seats of alighting passengers before newly boarding passengers can fill any remaining seats. At the boarding point passengers are assumed to mingle, meaning that FIFO is not observed, as is the case for many crowded bus and metro stops, particularly in European countries. The route choice considers the common lines problem and an user equilibrium solution is sought through a Markov type network loading process and the method of successive averages. The model is first illustrated with a small example network before being applied to the inner zone of London’s underground network. The effect of different values passengers might attach to finding a seat are illustrated. Applications of the model for transit planning as well as for information provision at the journey planner stage are discussed.     

Traffic assignment by paired alternative segments

The static user-equilibrium (UE) traffic assignment model is widely used in practice. One main computational challenge in this model is to obtain sufficiently precise solutions suitable for scenario comparisons, as quickly as possible. An additional computational challenge stems from the need in practice to perform analyses based on route flows, which are not uniquely determined by the UE condition. Past research focused mainly on the first aspect. The purpose of this paper is to describe an algorithm that addresses both issues. The traffic assignment by paired alternative segments (TAPAS) algorithm, focuses on pairs of alternative segments as the key building block to the UE solution. A condition of proportionality, which is practically equivalent to entropy maximization, is used to choose one stable route flow solution. Numerical results for five publicly available networks, including two large-scale realistic networks, show that the algorithm can identify highly precise solutions that maintain proportionality in relatively short computation times.     

Optimal traffic assignment to urban streets with air pollution consideration

Traffic assignment is usually determined solely on the basis of minimum travel time through the network. The present study on traffic assignment has taken into account not only traffic performance but also air quality over the street. A simple model of highway air pollution is developed by considering macroscopic material balance of polluted air mass over a segment of a highway that passes through an urban area, A new traffic assignment scheme has been developed based on the air pollution model. The optimal traffic assignment obtained by the new scheme is affected significantly by meteorological conditions.     

Assessment of air quality near traffic intersections in Bangalore city using air quality indices

Air quality indices are used for local and regional air quality management in many metro cities of the world. In the present study, air quality indices have been calculated using the US Environmental Protection Agency procedure to assess the status of ambient air quality near busy traffic intersections in Bangalore, India. The measured 24 h average criteria pollutants such as sulfur dioxide, oxides of nitrogen, respirable suspended particulate matter and suspended particulate matter for the period from 1997 to 2005 at three air quality monitoring stations are used for the development of AQIs. The result indicated that the air pollution at all the three air quality monitoring stations can be characterized as ‘good’ and ‘moderate’ for SO₂ and NO_x concentrations for all days from 1997 to 2004. Analysis of air quality indices values for both forms of suspended matter concentrations during 1999–2005 indicates 91% and 94% of the times days are in category ‘good’ and ‘moderate’. The yearly average air quality indices values of respirable suspended particulate matter and suspended particulate matter concentrations indicated decreasing trend and are coming under the category of ‘good’ and ‘moderate’ form the category of ‘poor’ and ‘very poor’.     

Traffic assignment in a two-dimensional continuous representation of a traffic network with flow-dependent speeds

The work deals with the assignment of traffic to a two-dimensional continuous representation of a traffic network. An important aspect of the treatment is that the reciprocal of the speed on each road in the network is at all times a linear function of the flow on that road. This speed-flow relationship is generalized to two-dimensional space using travel intensities and taking account of road densities, so that there is direct dependence of speeds upon flows at all points regardless of their location. There is also dependence of flows upon speeds at all points because Wardrop's first assignment principle is adopted. That is, for a given O-D pair, journey times on all routes actually used are identical, and less than journey times on all other possible routes. This results in the identification for each O-D pair of an “assignment zone”, an area within which all trips between that O-D pair are made, and beyond which no such trips are made. For a single O-D pair the assignment zone is identified by ?_m, the maximum angular divergence of a path from the straight line between O and D. Paths are then assumed to be bilinear so that for a single O-D pair the assignment zone is a parallelogram. Journey times, speeds, lateral displacement and other related quantities are obtained as functions of the flow Q between O and D. The work is extended to three O-D pairs located at the extremities of an equilateral triangle and four O-D pairs located at the corners of a square. At low flows these two configurations are trivial extensions of the single O-D pair problem because assignment zones do not overlap. At higher flows account is taken of this tendency to overlapping, so that although they do not overlap they do touch, becoming kite-shaped. Origins and destinations are assumed to be at the periphery of small circles of arbitrary radius. The work is inelegant to the extent that it involves a numerical integration but it is possible that this might eventually be circumvented.     

Long-memory characteristics of urban roadside air quality

Carbon monoxide is a major contributor to air pollution in urban cities, particularly at the roadside. Hourly, monthly and seasonal mean carbon monoxide concentration data are collected from a roadside air monitoring station in Hong Kong over 7-years. The station is a few metres from a major intersection surrounded by tall buildings. In particular, hourly patterns of concentrations on different days of the week are investigated. The data show that hourly carbon monoxide concentrations resemble the traffic pattern of the area and tend to be lower in the summer. Using a seasonal autoregressive integrated moving average models shows that the daily traffic cycle strongly influences concentrations. Further, it is found that urban roadside carbon monoxide monitoring data exhibits a long-memory process, suggesting that a model incorporating long memory and seasonality effects is needed simulate urban roadside air quality.     

Categorisation for air quality assessment in car cabin

The objective of this work was to propose a way of preparing information about in-cabin air quality for the car driver or passenger. It was assumed that information should be objective, relevant, current, complete and useful. The major elements of our concept were: (1) monitoring of selected air parameters inside the cabin, (2) determination of indicators that characterise indoor air, based on measurements, and (3) the method of information extraction. We proposed to realise the first task with a sensor system. The second and third goals were reached by categorisation. The basis for defining categories were ranges of values of measured parameters or their combinations. In a way, the categories were used to quantify indicators of air quality. Shannon entropy and mutual information were applied to find the best categorisation. The concept was investigated using experimental data from car cabin air monitoring in various driving conditions. We conclude that information about air quality in car cabins may be successfully conveyed using the following indicators: thermal conditions – determined based on temperature and relative humidity measurements; air exchange – determined based on CO₂ concentration; and air freshness – determined based on volatile organic compounds (VOCs) content in car cabin air. Each indicator has three categories: bad, intermediate and good.     

Traffic assignment by trip type using volume restraint and link restraint for application in small urban areas

This research involved the development of a new traffic assignment model consisting of a set of procedures for an urbanized area with a population of 172,000. Historical, social, and economic data were used as input to conventional trip generation and trip distribution models to produce a trip table for network assignment. This fixed table was divided into three trip types: external-external trips, external-internal trips, and internal-internal trips. The methodology used to develop the new traffic assignment model assigned each of the trip types by varying the diversion of trips from the minimum path. External-external trips were assigned on a minimum path routing and external-internal trips were assigned with a slight diversion from the minimum path. Internal-internal trips were assigned with more diversion than external-internal trips and adjusted by utilizing iterative volume restraint and incremental link restraint. A statistical analysis indicated that assigning trips by trip types using trip diversion and volume and link restraint produces a significant improvement in the accuracy of the assigned traffic volumes.     

Impact of Visible-Solar-Light-Driven photocatalytic pavement on air quality improvement

Photocatalytic pavement has attracted significant interest in the past decades by both the academia and industry for its ability of spontaneous cleaning of air pollutants, such as motor vehicle exhaust gas. Titanium dioxide (TiO₂) is used as the photocatalyst that is mixed into pavement materials or coated on the pavement to remove motor vehicle exhaust gases, e.g., carbon monoxide (CO), nitrogen oxides (NO_x), under the irradiation of the solar light. However, the pure TiO₂ additive only absorbs the light within the ultraviolet region due to its large bandgap. One approach to increase the ability of TiO₂ to the utilization of the full spectrum of the solar light is doping TiO₂. Therefore, this visible-solar-light-driven photocatalytic pavement embedded with doped-TiO₂ will exhibits a better cleaning efficiency of exhaust gas. This work conducted computational simulations of the cleaning efficiency on reducing exhaust gas NO₂ by photocatalytic pavement with doped-TiO₂, and its subsequent influence on the air quality in the surrounding environment. A three-dimensional model was developed for a section of pavement and its vicinal region. Effects of weather conditions, doped-TiO₂ coverages, road widths and traffic flow conditions on the removal of NO₂ and its influence to the adjacent environment were studied. Results indicate that visible-solar-light-driven photocatalytic pavement with doped-TiO₂ features a significantly higher removal efficiency of exhaust gas compared with the normal photocatalytic pavement. Moreover, the doped-TiO₂ embedded pavement is effective to remove NO₂ with different traffic densities and wind conditions, and consequently improve the air quality of the surrounding environment.     

Improvement of air quality by means of transportation system management

Under the United States federal Clean Air Act Amendments of 1977, states must implement transportation system management (TSM) tactics in urban areas that have not attained national ambient air quality standards for carbon monoxide and photochemical oxidants. This paper provides a preliminary assessment of the effectiveness and feasibility of using TSM tactics to improve air quality. Based on this assessment, the authors conclude that TSM measures should be effective in eliminating localized carbon monoxide problems, but that such measures are not likely to contribute significantly toward reducing regional oxidant levels. In addition, because most individual TSM tactics can have only marginal impacts on regional motor vehicle emissions, coordinating the planning and implementation of a portfolio of TSM measures will be an essential element of an effective TSM program for improving air quality.     

Meeting mobility and air quality goals: Strategies that work

Recent federal legislation sets tough air quality goals for the nation but offers scant guidance to urban areas responsible for meeting those goals. Traditionally, transportation-related air quality policy emphasized cleaner vehicle and fuels technologies and alternatives to single-occupant vehicle travel such as car-pooling and mass transit. Analysis suggests, however, that meeting air quality goals will require the addition of policies to manage the growing demand for transportation. The introduction of market forces into transportation supply and demand decisions would support traditional transportation and air quality strategies, and produce additional mobility, air quality, and economic benefits. Recent transportation legislation offers states and localities the flexibility to meet mobility and air quality goals in an innovative, nontraditional manner. Drawing on theoretical analysis and recent empirical evidence we offer a package of measures for the consideration of state, and federal policy-makers.The authors are with the Office of Policy Analysis of the U.S. Environmental Protection Agency. The views expressed here belong solely to the authors. This paper does not represent the view of the US EPA or the policy of the United States Government. This paper was significantly improved by extensive comments from Bruce Schillo, and by suggestions from Michael Shelby, John Chamberlin (all of the EPA Office of Policy Analysis), Robin Miles-McLean (EPA Office of Mobile Sources), Allen Basala (EPA Office of Air Quality Planning and Standards), and numerous other colleagues. The authors are responsible for all errors and interpretations.     

Indoor air quality and passenger thermal comfort in Beijing metro transfer stations

In this study, particulate matter was investigated as the primary pollutant in the air quality of Beijing Metro transfer stations, and passenger thermal comfort during the transfer process was evaluated by using the relative warmth index (RWI). Passenger thermal comfort level is not ideal in 87% of the measured space and is slightly hot overall, with an RWI range of 0.20–0.43. Although 20% of the measured space has lower values than ASHRAE’s cooling comfort class, the thermal comfort level of most measured space is good in winter morning rush hours, with an RWI range from −0.18 to 0.28. The particulate matter (PM) concentration is related not only to the season and spatial depth but also to the transfer design of the metro station. During the morning rush period, the concentration ranges difference of PM₁₀ and PM_2.5 in winter are 262.9 μg/m³ and 125.5 μg/m³, respectively, which are 1.43 and 1.46 times higher than those of in summer. There are significant differences in the PM concentration and RWI values between the island and lateral platforms of Beijing Metro transfer stations, and the design of the lateral platform is superior to that of the island platform. Another exploratory experiment is conducted to determine if the PM concentration has a potential effect on human metabolic rate. The data in this paper provide a valuable reference for further comfort research and environmental control in metro station, and the conclusions may guide the further underground space design of metro transfer stations.     

Evaluating critical lines and stations considering the impact of the consequence using transit assignment model ‐case study of London's underground network

As the problem of full transit vehicles is encountered daily by passengers in most of the big cities, previous research evaluated the consequence of overcrowding in terms of on‐board crowding and passengers not being able to board with full vehicles. The impact of overcrowding in the real world is, however, not necessarily proportional to these numbers. This paper attempts to specify the critical lines and stations of a network by considering the number of passengers failing to board and attempting to evaluate its impact on service quality and safety risks. The hypothesis is that larger stations with wider platforms can often cope better with overcrowding than smaller stations. Therefore a station size dependent satisfaction function is proposed, which takes values from 0 to 1. The method is applied to London's underground network with a number of scenarios which show critical stations in the network if delays occur.     

Rail freight traffic assignment

This paper describes an algorithm for routing freight over a rail network whose tracks are controlled by several carriers in a manner that follows current industrial practices. In particular, the freight is routed to minimize the number of interline transfers and to maximize the revenue division for the originating carrier. The algorithm is based upon generalizing a shortest path algorithm to allow vector valued link impedances.     

Learning from Los Angeles: transport,urban form,and air quality

Los Angeles is well known around the world as an automobile-oriented low density community, yet recent transportation policies have emphasized greater capital investment in rail transportation than in highways, and recent policies have attempted to discourage automobile usage through transportation demand management. While these policies have accomplished small shifts toward public transport and somewhat lower dependence upon singly occupied automobilies for work commuting, the financial costs of these policy changes has been very large in relation to their benefits. Proper pricing of transportation alternatives, more creative use of new and emerging transportation technologies, and the provision of many more opportunities for simpler private sector transport services, would all appear to be more promising as cost-effective approaches to coping with congestion in Los Angeles than the current regional transportation policies.     

Short/medium-term prediction for the aviation emissions in the en route airspace considering the fluctuation in air traffic demand

This paper proposes a novel short/medium-term prediction method for aviation emissions distribution in en route airspace. An en route traffic demand model characterizing both the dynamics and the fluctuation of the actual traffic demand is developed, based on which the variation and the uncertainty of the short/medium-term traffic growth are predicted. Building on the demand forecast the Boeing Fuel Flow Method 2 is applied to estimate the fuel consumption and the resulting aviation emissions in the en route airspace. Based on the traffic demand prediction and the en route emissions estimation, an aviation emissions prediction model is built, which can be used to forecast the generation of en route emissions with uncertainty limits. The developed method is applied to a real data set from Hefei Area Control Center for the en route emission prediction in the next 5 years, with time granularities of both months and years. To validate the uncertainty limits associated with the emission prediction, this paper also presents the prediction results based on future traffic demand derived from the regression model widely adopted by FAA and Eurocontrol. The analysis of the case study shows that the proposed method can characterize well the dynamics and the fluctuation of the en route emissions, thereby providing satisfactory prediction results with appropriate uncertainty limits. The prediction results show a gradual growth at an average annual rate of 7.74%, and the monthly prediction results reveal distinct fluctuation patterns in the growth.