4,637 research outputs found

    Implementation of connected and autonomous vehicles in cities could have neutral effects on the total travel time costs: modeling and analysis for a circular city

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    Autonomous vehicles promise to revolutionize the automobile market, although their implementation could take several decades in which both types of cars will coexist on the streets. We formulate a model for a circular city based on continuous approximations, considering demand surfaces over the city. Numerical results from our model predict direct and indirect effects of connected and autonomous vehicles. Direct effects will be positive for our cities: (a) less street supply is needed to accommodate the traffic; (b) congestion levels decrease: travel costs may decrease by 30%. Some indirect effects will counterbalance these positive effects: (c) a decrease of 20% in the value of travel time can reduce the total cost by a third; (d) induced demand could be as high as 50%, bringing equivalent total costs in the future scenario; (e) the vehicle-kilometers traveled could also affect the future scenario; and (f) increases in city size and urban sprawl. As a conclusion, the implementation of autonomous vehicles could be neutral for the cities regarding travel time costs. City planning agencies still have to promote complementary modes such as active mobility (walking and bicycle), transit (public transportation), and shared mobility (shared autonomous vehicles and mobility as a service).Peer ReviewedPostprint (published version

    Farmers' management of rice varietal diversity in the mid-hills of Nepal: implications for on-farm conservation and crop improvement

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    Season-long monitoring of on-farm rice (Oryza sativa, L.) plots in Nepal explored farmers' decision-making process on the deployment of varieties to agroecosystems, application of production inputs to varieties, agronomic practices and relationship between economic return and area planted per variety. Farmers deploy varieties [landraces (LRs) and modern varieties (MVs)] to agroecosystems based on their understanding of characteristics of varieties and agroecosystems, and the interaction between them. In marginal growing conditions, LRs can compete with MVs. Within an agroecosystem, economic return and area planted to varieties have positive relationship, but this is not so between agroecosystems. LRs are very diverse on agronomic and economic traits; therefore, they cannot be rejected a priori as inferior materials without proper evaluation. LRs have to be evaluated for useful traits and utilized in breeding programmes to generate farmer-preferred materials for marginal environments and for their conservation on-farm

    Symplectic integrators in the shearing sheet

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    The shearing sheet is a model dynamical system that is used to study the small-scale dynamics of astrophysical disks. Numerical simulations of particle trajectories in the shearing sheet usually employ the leapfrog integrator, but this integrator performs poorly because of velocity-dependent (Coriolis) forces. We describe two new integrators for this purpose; both are symplectic, time-reversible and second-order accurate, and can easily be generalized to higher orders. Moreover, both integrators are exact when there are no small-scale forces such as mutual gravitational forces between disk particles. In numerical experiments these integrators have errors that are often several orders of magnitude smaller than competing methods. The first of our new integrators (SEI) is well-suited for disks in which the typical inter-particle separation is large compared to the particles' Hill radii (e.g., planetary rings), and the second (SEKI) is designed for disks in which the particles are on bound orbits or the separation is smaller than the Hill radius (e.g., irregular satellites of the giant planets).Comment: 9 pages, 6 figures, accepted for publication in MNRAS, v2: discussion/tests for symmetrized and modified leapfrog integrators adde

    IMPACTS OF INTERNATIONAL MAIZE BREEDING RESEARCH IN DEVELOPING COUNTRIES, 1966-98

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    This report, which updates and extends the findings of an earlier CIMMYT study published in 1994, documents the impacts of international maize breeding research in the developing world. Covering the period 1966-98, the report reviews public and private investment in maize breeding research, describes the products of public and private maize breeding programs, estimates farm level adoption of modern varieties (MVs), and estimates the gross value of additional grain production attributable to international breeding efforts. Although private companies have greatly increased their investment in maize breeding research in recent years, public maize breeding programs still play an important role, especially in breeding for subsistence-oriented farmers. Seed sales data show that the maize seed industry in many developing countries has effectively been privatized and that hybrid seed sales now dominate sales of all other seed types. The area planted to MVs continues to expand at an impressive rate. Maize MVs are currently grown on at least 58.8 million ha in developing countries, including at least 21.2 million ha planted to MVs that contain CIMMYT germplasm. The gross value of additional grain production attributable to the adoption of maize MVs in developing countries is estimated to range from US3.7milliontoUS 3.7 million to US 11.1 billion per year. Analysis of varietal pedigrees shows that breeders in both the public and private sectors have made extensive use of CIMMYT germplasm. Over 54% of publicly bred MVs released in the developing world since 1966 have contained CIMMYT germplasm. The pedigrees of many privately bred cultivars are confidential, but CIMMYT germplasm was present in 58% of MVs developed by private breeding programs being sold in the late 1990s for which pedigree information is available. The gross benefits attributable to CIMMYT's maize breeding program are estimated to range from US167milliontoUS 167 million to US 1.5 billion per year.Productivity Analysis,

    Portfolio Performance Gauging in Discrete Time Using a Luenberger Productivity Indicator

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    This paper proposes a pragmatic, discrete time indicator to gauge the performance of portfolios over time. Integrating the shortage function (Luenberger, 1995) into a Luenberger portfolio productivity indicator (Chambers, 2002), this study estimates the changes in the relative positions of portfolios with respect to the traditional Markowitz mean-variance efficient frontier, as well as the eventual shifts of this frontier over time. Based on the analysis of local changes relative to these mean-variance and higher moment (in casu, mean-variance-skewness) frontiers, this methodology allows to neatly separate between on the one hand performance changes due to portfolio strategies and on the other hand performance changes due to the market evolution. This methodology is empirically illustrated using a mimicking portfolio approach (Fama and French 1996; 1997) using US monthly data from January 1931 to August 2007.shortage function, mean-variance, mean-variance-skewness, efficient portfolios, Luenberger portfolio productivity indicator

    Infrastructure and technology constraints to agricultural development in the humid and subhumid tropics of Africa:

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    Green Revolution technologies have not been widely adopted in Africa. What are the chances that they will play a major role in the near future? This paper shows that the enabling infrastructure, especially rural roads and irrigation systems are not likely to be in place in the humid and sub-humid tropics of Africa in the next 20-30 years. Consequently a typology of the more appropriate set of technology that is input and infrastructure efficient, has high returns to seasonal labor and is sustainable is presented. Research institutions should be geared up now to produce them for use in 10-20 years.Green Revolution Africa., Infrastructure (Economics), Rural development Africa., Technological innovations., Irrigation., Africa.,
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