A variational formulation of kinematic waves: Solution methods

This paper presents improved solution methods for kinematic wave traffic problems with concave flow-density relations. As explained in part I of this work, the solution of a kinematic wave problem is a set of continuum least-cost paths in space-time. The least cost to reach a point is the vehicle number. The idea here consists in overlaying a dense but discrete network with appropriate costs in the solution region and then using a shortest-path algorithm to estimate vehicle numbers. With properly designed networks, this procedure is more accurate than existing methods and can be applied to more complicated problems. In many important cases its results are exact.     

Topological constraints on the dynamics of wasp-waist ecosystems

Small pelagic fish species like anchovy or sardines are of high ecological and economical importance. As marine food webs are fished down, these small pelagics tend to be more exploited and overfished. It is not yet very well known what the possible effects of their collapse can be, therefore there is an urgent need to outline a theoretical framework for understanding their dynamics. These fish occupy very special position in food webs, ensuring energy transfer between species lower and higher levels, while forming narrow “wasp-waists” poor in number of species (but very abundant). Our purpose was to quantify the interaction structure of model food webs of equal complexity but different levels of “wasp-waistedness”. We analysed the topological properties of the webs by characterising every direct and indirect interactions between individual species, as well as by assessing the relative positional importance of each species in each web. We found that (1) the shorter the interaction pathways considered, the weaker the predictive power of node degree for positional importance, (2) the importance of species varies more in wasp-waist food webs, (3) if longer indirect chain effects are considered, indirect effects can well be stronger than direct ones, (4) interactions between coexisting wasp-waist species are stronger than the average, and (5) the “self-regulatory” looping effects are also stronger for wasp-waist species. Based on the topological properties of the networks, our results describe constraints acting on the dynamical behaviour of wasp-waist ecosystems. We give explanations, from this viewpoint, for regime shifts in which one WW species replaces another, and for the unpredictable dynamics of these fish stocks. From a marine conservation viewpoint, we illustrate that as the abundance of wasp-waist species decreases, the architecture of energy flows becomes highly vulnerable and unreliable. We provide an approach for quantifying these structural changes.     

An analytical approximation for the macroscopic fundamental diagram of urban traffic

This paper shows that a macroscopic fundamental diagram (MFD) relating average flow and average density must exist on any street with blocks of diverse widths and lengths, but no turns, even if all or some of the intersections are controlled by arbitrarily timed traffic signals. The timing patterns are assumed to be fixed in time. Exact analytical expressions in terms of a shortest path recipe are given, both, for the street’s capacity and its MFD. Approximate formulas that require little data are also given.For networks, the paper derives an upper bound for average flow conditional on average density, and then suggests conditions under which the bound should be tight; i.e., under which the bound is an approximate MFD. The MFD’s produced with this method for the central business districts of San Francisco (California) and Yokohama (Japan) are compared with those obtained experimentally in earlier publications.     

A subadditivity test for the cost function of the principal European railways

The objective of this paper is the comparison of the efficiency levels for a set of European rail companies in monopoly situations with respect to hypothetical duopoly situations. More specifically, the aim is to test whether the operating costs function of such companies is subadditive. A cost function is subadditive when a single firm can produce all relevant output vectors cheaper than two or more firms. In order to do so the test developed by Evans and Heckman (1984) will be used. It was applied for the American phone company Bell System to assess the convenience of separating the management of local and long-distance services. The results indicate that at least for the companies of a greater size, an efficiency increase (as measured by lower costs), would be obtained from the separation of the current companies in two different companies of a lower size. Besides, these efficiency gains are greater when the two new companies specialize their production, either on passenger or on freight traffic.     

Who switches to hybrids? A study of a fuel conversion program in Colombia

Air pollution from mobile sources is an important environmental problem in larger cities. In 2001, a program was implemented to encourage the use of natural gas in vehicles in the Aburrá Valley in Colombia, with incentives to convert small cars from gasoline and diesel to hybrid engines with natural gas, most notably a cash subsidy. Using a survey administered to both commercial and private car owners we study the determinants of conversion under this fuel conversion program. We thus obtain information about the reasons for adoption of new technologies in vehicles. This allows us to discuss the possible outcomes of this type of policy. Results show that a large part of owners who switched would have done it anyway without the subsidy. Based on the findings, commercial vehicles are most likely to be converted to natural gas vehicles.     

Enjoyment of commute: A comparison of different transportation modes

This study investigates how socio-demographic and attitudinal variables of university students affect their desire to increase or decrease their daily commute. The case study is McMaster University in Hamilton, Canada, and data was obtained by means of a web-based survey that included questions regarding travel behavior, socio-demographic information, and attitudes toward travel, land use, and the environment. The objective variable is defined as the ratio of ideal to actual commute time, and regression analysis is implemented to test the relationship between this variable and socio-demographic variables and attitudinal scores. The impact of different attitudes on the gap between ideal and actual commute time is expanded to include three different modes, active travel (walk/cycle), transit, and personal automobile. Interestingly, the results indicate that active travelers tend to be less dissatisfied with their commute, followed by those who travel in a personal vehicle and transit users. A number of attitudinal responses are shown to impact the desire to travel more or less, including variables that relate to the social environment, availability of local activities, quality of facilities, productive use of the commute, and the intrinsic value found in the commute travel. The picture emerges of a traveler who would like to spend more time commuting, as someone who is an active traveler, thinks that getting there is half the fun, dislikes traveling alone, but rather likes to live in an active neighborhood where there is a sense of community. The results suggests that enjoyment of commuting, while a challenge from the perspective of motorized mobility, may provide valuable policy opportunities from the perspective of active transportation.     

Analyzing the structure of informal transit: The evening commute problem

Through the use of a profit-maximizing continuum approximation model, this paper systematically analyzes the development and structure of informal transit systems as a function of the network, user, and modal characteristics. This study examines the evening commute problem along a linear corridor where passengers originate uniformly from a central business district and have destinations uniformly distributed along the corridor. Informal transit drivers who are profit-maximizing will be compared against the traditional case of coordinated, government service that aims to maximize the total welfare. Policies, such as fare regulation and vehicle licensing schemes, will be presented to help rationalize informal transit service using a government-operated service as the baseline.     

The influence of three-way catalysts on harmful emission production

This article deals with the influence of three-way catalysts on the production of basic emissions, such as carbon monoxide, unburned hydrocarbons and nitrogen oxides from spark-ignition engines. A virtual simulation of the new European driving cycle is used. Characteristics of components in emissions in the front and back of the catalyst are measured on the test bed to form the basis of the simulation. The results relate to emissions for 1 km travelled.     

Assessment of hybrid‐drive bus fuel savings for Brazilian urban transit

Buses are the main transit mode in Brazil, transporting more than 55 million passengers per day. Most of these vehicles run on diesel oil causing a dependence on oil, extensive greenhouse gas emissions and increasing air pollution in urban areas. In order to improve this situation, options for Brazilian cities include the use of alternative fuels and new propulsion technologies, such as hybrid vehicles. This paper proposes a procedure for evaluating the performance of a recently developed hybrid‐drive technology. A simple procedure is presented to compare hybrid‐drive buses with conventional diesel buses in urban operations, particularly with respect to fuel economy. Next the potential for reducing diesel oil consumption through the use of hybrid‐drive buses is assessed. Field tests carried out by the authors indicate that fuel consumption improvement through the use of hybrid‐drive buses would certainly exceed 20%, resulting in lower fuel costs and carbon dioxide (CO₂) emissions.     

马德里:城市地下公路与铁路基础设施安全高效发展的全球典范

马德里已经成为城市地下公路与铁路基础设施安全高效发展的一个全球典范.作为改善城市交通系统与公共运输网络的有效投资典范,文章详细介绍了过去12年中地铁网络的发展以及目前M-30环线改建项目情况,分析总结了马德里获得成功的关键因素.