Interval constraints for the treatment of the partial data problem in gravity model calibration

The problem of indeterminacy, in the solution parameters of the Evans-Kirby calibration model, is discussed. A proposal is made to alleviate this problem, in the case of missing data, by introducing interval constraints.     

Development and Simulation of a Novel Roll Control System for the Interconnected Hydragas® Suspension

The design of passive suspension systems using conventional springs and dampers is limited by the need to compromise between vehicle ride and handling functions. The Interconnected Hydragas Suspension fitted to the current Rover 100 series partially allays this compromise by reducing the vehicle pitch stiffness witfiout affecting the bounce and roll stiffnesses. However, the vehicle body is still subject to roll during cornering manoeuvres. This paper outlines the development and simulation of a sealed low bandwidth active roll control suspension based on the existing Interconnected Hydragas System. Following a brief explanation of the Hydragas suspension operating principle die paper outlines the design of a fluid displacer or 'shuttle'. This shuttle enables control over body roll during manoeuvres by displacing fluid from one side of the car to the other. Care is taken to ensure low power consumption whilst the sealed nature of the fluid based suspension units guarantee reliable operation without leakage. Using computer simulation, the system performance is predicted and compared with experimental measurements. It is shown that roll during manoeuvres can be reduced or eliminated using a minimum of hydraulic components with only moderate power consumption and cost.     

Bus journey times in medium size urban areas

The predictive accuracy of the models based on the fundamental relation between journey time and passenger demand can be improved through data disaggregation or route segmentation. Primary reason for this is the improvement in the estimates of stopping delays and delays due to passenger boarding and/or alighting (dwell time). Both Poisson and Negative Binomial model estimates of stoppings for passenger boarding and alighting are shown to improve with disaggregation. These improvements, however, contribute little to the overall predictability of the fundamental models which are useful for gaining insight into the significance and variability of the stopping delays and dwell time, or testing sensitivity to changes in the long term. Site or route specific models of journey times which have better predictive capability exist, and may be used for short-run planning. However, the interchangeability and performance over time of the latter, have to be evaluated before making definitive conclusions.     

Characteristics of truck traffic in Singapore

This paper presents the results of a project conducted to study the characteristics of truck traffic in Singapore. Detailed traffic surveys recording counts of vehicles by axle-configuration were performed at 219 sites over a period of nearly two years. The surveys covered 5 different road classes, namely expressways, arterials, collectors, industrial roads and local roads. It was found that the time distribution of truck travel were not the same among the five road classes. The peaking characteristics of truck traffic were less pronounced compared to passenger car traffic. The peak hour truck volume varied from 67.0% to 9.7% of the daily truck traffic as compared to 13.8% for passenger car traffic. The lane distribution pattern of truck traffic was studied in detail by road class, and was found to be a function of total directional traffic volume, total directional truck volume and the number of traffic lanes. Composition analysis was also carried out to study the lane use characteristics of single- and multiple-unit trucks.     

Heuristic ranking and selection procedures for network design problems

Two heuristic approaches to solving discrete non-linear network design problems and other subset selection problems are described and tested. These heuristics operate similarly to other add, delete and interchange heuristics that have been applied to network design problems. However, the selection criterion adopted here is the ratio of objective function change per unit of resource cost, which other authors have described as an effective gradient measure for zero-one integer programming problems. Optimal branch-and-bound algorithms previously developed and tested are reviewed, and those results serve as benchmarks with which to compare these heuristics described in this paper. The first heuristic ranks and deletes alternative network modifications from an entire set of candidate projects, such as new links or link improvements, until the subset of remaining projects fits within the constraint space. This method was found to achieve optimal or near-optimal solutions to each of the test cases, even when the number of candidate projects deleted in each iteration was increased from one to many. The quality of those results led us to investigate a constrained random sampling procedure in which candidate projects are ranked with regard to randomly generated networks, and a solution is then chosen on the basis of these rankings. This second approach was found to achieve solutions that were almost as good as those obtained with the rank and delete heuristic.     

The effect of round-off error on computed solutions of trip distribution problems

Round-off error analyses of two versions of the Kruithof method are given. It is shown than an alternative implementation is numerically superior to the usual implementation. Since it also has the advantage of requiring less writing in secondary memory, and requires the same number of arithmetic operations, the case for always using the alternative implementation seems indisputable.     

Fleet deployment optimization models. Part 1

The problem of minimum-cost operation of a fleet of ships that has to carry a specific amount of cargo between two ports in a given time period for a specific, fixed contract price is studied. Detailed and realistic operating cost functions are developed. Sensitivity analyses are performed to study the effects of small or large changes in one or more cost components on the total costs. A realistic model for the annual transport capacity as a function of speed is also used, in contrast with the linear relation most often used in the literature. The full load and ballast speeds for those ships of the fleet that we operate are selected to minimize the total fleet operating costs including lay-up costs for unused vessels, using nonlinear optimization algorithms.     

Simulator evaluation of incident detection by transponder-equipped vehicles

This paper explores the efficacy of the driver-based incident detection using the vehicle-to-roadside communication (VRC) system. The proliferation of vehicle tags in the US for automatic toll collection, traffic monitoring, and vehicle navigation and information systems has created an infrastructure capable of supporting a driver-based incident detection system. The research reported herein investigated the use of "activatable" vehicle tags by drivers to send an incident signal to the Traffic Management Center through VRC reader stations spaced uniformly on a highway. The simulation results showed that good detection performance was achieved even at lower levels of market penetration of vehicle tags. The results further showed that detection performance is significantly affected by the severity of the incident in terms of number of lanes closed, the spacing of the VRC reader stations, traffic volume at the time of the incident, and the reporting propensity of the traveling public.The performance of the VRC-based incident reporting system was compared to the performance of two incident detection algorithms that rely on traffic data collected through the automatic vehicle identification (AVI) system. The comparison showed that the VRC-based incident reporting system attained shorter detection times and higher detection rates under fairly similar simulated conditions. The paper also discusses issues that need further study through simulation and field experimentation of the VRC-based incident reporting system.     

Designing H∞/GH 2 static-output feedback controller for vehicle suspensions using linear matrix inequalities and genetic algorithms

This paper presents an approach to design the H_∞/GH ₂ static-output feedback controller for vehicle suspensions by using linear matrix inequalities (LMIs) and genetic algorithms (GAs). Three main performance requirements for an advanced vehicle suspension are considered in this paper. Among these requirements, the ride-comfort performance is optimized by minimizing the H_∞ norm of the transfer function from the road disturbance to the sprung mass acceleration, while the road-holding performance and the suspension deflection limitation are guaranteed by constraining the generalized H₂ (GH ₂) norms of the transfer functions from the road disturbance to the dynamic tyre load and the suspension deflection to be less than their hard limits, respectively. At the same time, the controller saturation problem is considered by constraining its peak response output to be less than a given limit using the GH ₂ norm as well. A four-degree-of-freedom half-car model with active suspension system is applied in this paper. Several kinds of H_∞/GH ₂ static-output feedback controllers, which use the available sprung mass velocities or the suspension deflections as feedback signals, are obtained by using the GAs to search for the possible control gain matrices and then resolving the LMIs together with the minimization optimization problem. These designed H_∞/GH ₂ static-output feedback controllers are validated by numerical simulations on both the bump and the random road responses which show that the designed H_∞/GH ₂ static-output feedback controllers can achieve similar or even better active suspension performances compared with the state-feedback control case in spite of their simplicities.     

Interaction of vehicle and steering system regarding on-centre handling

For the on-centre handling behaviour of vehicles the steering system is absolutely important. To investigate the interaction of the vehicle and steering system a validated, especially tailored simulation model was developed. Some meaningful vehicle and steering system parameters are altered to show the influence on steering wheel torque, steering feel and understeer. The results underline the importance of an accurate steering system model. Identified measures to improve the centre feel and steering response were a stiffer torsion bar, a higher cornering stiffness or a lower overall steering ratio. The steering response, however, suffers when the centre feel is improved by a higher trail. The steering rack friction reduces mainly the steering response while the steering column friction decreases the centre feel whereas a stiffer torsion bar lessens the understeer tendency.