Francisco G. Benitez

ON THE FUNCTIONAL FORM OF THE SPEED-DENSITY RELATIONSHIP--I: GENERAL THEORY

In this work a functional form for the speed-density relationship is presented. This functional form is made up of a nondimensional spacing, the equivalent spacing and of a function, the generating function, whose argument is the equivalent spacing. This functional form is derived by means of two different arguments. The first argument is based on the set of properties that the volume-speed-density relationships should satisfy. The second one arises when applying a dimensional analysis to a generic car-following model. Finally, several examples of generating functions are shown.

On the functional form of the speed-density relationship--II: Empirical investigation

In this part we applied the theory developed in Part I. Some speed-density curves having the functional form proposed in Part I are fitted to traffic data. The goodness of fit is excellent, except in the case of the left lane detectors. A procedure for isolating stationary traffic periods is also explained. This procedure satisfactorily eliminates the dispersion of the individual measurements.

The reaction time of drivers and the stability of traffic flow

In this work, a condition for any traffic flow model is proposed: the consistency condition. An expression for the reaction time of drivers as a function of traffic density is derived from this condition. The changes introduced in the Payne model by the adoption of this expression for the reaction time are investigated. A comparison analysis of the resulting Payne model with others proposed by several authors and with the Simple Continuum Model is carried out. This analysis leads to the conclusion that the results of the Payne model are almost identical to those given by the Simple Continuum Model. Finally, the stability of traffic flow is studied by linearizing the Payne model and the car-following models modified by the adoption of the new formulation for the reaction time. The analysis shows that the inclusion of stochastic terms in the models would be necessary to explain instability phenomena in traffic flow.

Determining a public transport satisfaction index from user surveys

The quality of public transport systems may be directly observed through user surveys by collecting ratings given by the users to specific aspects such as network coverage, transfers between lines and waiting time, among others. Besides these specific satisfaction ratings, an overall global satisfaction rating of the service is required. This way of proceeding, by asking questions on a limited number of disaggregated aspects, makes it easier to analyse the factors involved in the quality of a means of public transport and to rank these factors according to their contribution to the global satisfaction. This article presents a methodology to determine the relationship between the global satisfaction rating and the specific satisfaction ratings. This methodology employs three types of models for such a relationship: weighted means, a multivariate discrete distribution and a generalised linear model. These models allow the identification of the contribution of the specific satisfaction ratings to the global satisfaction rating. This information may be used by transit companies to improve their service quality.

OPTIMIZATION FOR VEHICLE SUSPENSION II: FREQUENCY DOMAIN

SUMMARY The objective of this study is optimizing the components design of a vehicle suspension system under excitation due to road roughness. The vehicle is modelled as a dynamic system made of masses interconnected by, linear, springs and dampers. The optimizing code provides values corresponding to the caracteristics of masses, dampers and springs which, within a range, minimize the objective function for a defined excitation. This objective function auantifies the vehicle comfort level. The optimization method used is the sequential linear programming by iteratively applying the Simplex algorithm. The model response is obtained in frequency domain and the vehicle excitation can be either random or deterministic. The exact nature of the optimization problem, objective function and restrictions, depend on the type of excitation considered. In succeeding paragraphs, the problem formulation together with a comparison with other authors is presented.

Experimental investigation of the dynamic response of graphite-epoxy composite laminates under compression

Abstract In this study, the dynamic stress–strain response of graphite-epoxy composite laminates is investigated. The laminates are interposed in a section of a split Hopkinson apparatus. A quasi-rectangular wave is generated at one end of the incident bar by striking it with another bar of known length. This bar is accelerated using a compressed air gun. Approximate average stresses and strains can be obtained by measuring the incident, reflected and transmitted waves in the split bar. The dynamic behavior is evaluated for a range of impact velocities. The dependence of the response on impact velocity is analyzed and discussed. Three different specimen thicknesses have been used. These are obtained by increasing the repetition factor of a base stacking sequence: (+45°, −45°, 0°, 90°). This process is called sublaminate scaling; it is preferred to ply scaling since it has been shown that the accumulation of layers of the same orientation decreases the failure load to such an extent that residual stresses may crack the specimen before any external load is applied. The laminates considered are: (+45°, −45°, 0°, 90°) ns , n =2,3,4. The scale effects observed in the experimental response are analyzed and discussed.

Infinitely variable transmission of racheting drive type based on one-way clutches

An infinitely variable transmission (IVT), based on the use of one-way action clutches, belonging to the family of ratcheting drives is described. The mechanical foundations and numerical simulations carried out along this research envisage a plausible approach to its use as gear-box in general mechanical industry and its prospective use in automobiles and self-propelled vehicles. The system includes one-way clutches-free wheels or overrunning clutches-and two epicyclic gear systems. The output velocity, with oscillatory character, common to the ratcheting drives systems, presents a period similar to that produced by alternative combustion motors, making this transmission compatible with automobile applications. The variation of the transmission is linear in all the working range. The kinematics operating principles behind this IVT is described followed by a numerical simulation of the dynamic analysis. A prototype has been constructed and tested to assess its mechanical efficiency for different reduction ratios. The efficiency values predicted by theory agree with those experimentally obtained on a bench-rig testing equipment.

Elastic stress and displacement distribution in an orthotropic multilayered system due to a concentrated load

A method to calculate the fundamental solution of an infinite three-dimensional, linear, elastic, orthotropic, homogeneous multilayered system due to a concentrated load acting in the interior of an arbitrary layer is presented. It was found that an analytical solution does not exist due to the complexity of the eigenvalue solution of the problem. In order to compute the solution a seminumerical method was developed and the resulting equations were solved using the fast Fourier transform (FFT). This paper is based on the work developed in a companion paper (Wideberg and Benitez, Elastic stress and displacement distribution in an orthotropic layer due to concentrated load) for the fundamental solution of an orthotropic three-dimensional layer. Numerical results are presented for a graphite epoxy fibre-reinforced composite plate.

Improving trip forecasting models by means of the Box–Cox transformation

This article presents a methodology for improving the performance of a trip generation/attraction forecasting model by means of the Box–Cox transformation. The application of this transformation to a set of random variables aims at obtaining a set of transformed variables distributed normally. The methodology comprises two steps. First, the sample is transformed employing the parameters of the transformation that maximises the likelihood as a multivariate normal distribution. In the second step, the forecast of the dependent variable, that is, the number of trips, is produced by an analytical approximation for the mean value of that variable conditioned on the rest of the variables. The methodology is applied to three different samples consisting of road trips and of a series of socio-economic variables. The analysis of the results shows that the model based on the optimal Box–Cox transformation has a better forecasting performance than that based on the logarithmic transformation.

Continuous Variable Transmission With an Inertia-Regulating System

This article describes a power transmission system applicable to vehicles. It consists of an oscillating, ratcheting-type, continuously variable transmission (CVT) system governed by an inertia mechanism. The inertia-regulating mechanism adds an additional degree of freedom and gives the system a dynamic character. The transmission consists of three different subsystems. The first of these converts the rotation of the engine or motor into an oscillating angular velocity movement and regulates the amplitude of this movement. The oscillating rotation from the first subsystem is used to drive a second subsystem, which acts as a regulating device by means of an inertial mechanism. The oscillating movement at the output of the second subsystem is rectified in the third, resulting in a unidirectional angular velocity. As a result, a unidirectional torque is generated at the output of the CVT, commensurate with the operating condition of the transmission, and this is capable of overcoming a torque resistance. A prototype of this transmission was built and tested to check the experimental results against those predicted by a series of computational simulations. As a result, the experimental graphs that characterize the operation of this type of transmission system were obtained, demonstrating its ability to function in an efficient manner.