Transit performance evaluation in the U.S.A.

Performance of transit agencies in the United States improved during the 1980s. At the beginning of the decade, Americans had become disenchanted with transit; legislation was passed that required agencies to report performance and accept regular audits. Theory underlying these policies is examined in four components: dimensions for policy objectives, indicators, information systems and incentives. Three programs are examined: federal triennial reviews that monitor compliance with planning and grant requirements, California performance audits that analyze goals and track performance on five indicators and the Los Angeles program that encourages improvement by offering incentive payments for better-than-average performance. The California audits have been the most successful.     

A note on goodness-of-fit statistics for probit and logit models

In the case of models designed to explain the choice among a finite set of alternatives, a number of goodness-of-fit statistics have been reported. This paper is primarily concerned with the properties of one of these statistics, the likelihood ratio index.By comparing the likelihood ratio index with some of the other statistics and by examining its mathematical properties, it is concluded that the index has desirable properties in binary and multinomial situations. However, the way in which the likelihood ratio index has been applied in many recent studies has led to results which are possibly unexpected. In these cases, the index was a measure of the extent to which a hypothesized model improved upon the explanatory power of a model with all coefficients, including the constant or the coefficients of alternative-specific dummies, equal to zero. It is shown that the minimum value of this likelihood ratio index depends on the relative proportions of sampled individuals selecting the various alternatives, contrary to the expectation of a zero minimum value. The dependence on the sampled proportions also prevents comparison of indices resulting from different samples.A simple adjustment alleviates these difficulties. This new definition makes the likelihood ratio index a measure of the extent to which the hypothesized model improves upon the explanatory of a model with only a constant or alternative-specific dummies. It is recommended that this index is more appropriate for assessing the value of choice models.A list of symbols used is given on page 387.     

Optimal control of brakes and steering for autonomous collision avoidance using modified Hamiltonian algorithm

ABSTRACT

This paper considers the problem of collision avoidance for road vehicles, operating at the limits of friction. A two-level modelling and control methodology is proposed, with the upper level using a friction-limited particle model for motion planning, and the lower level using a nonlinear 3DOF model for optimal control allocation. Motion planning adopts a two-phase approach: the first phase is to avoid the obstacle, the second is to recover lane keeping with minimal additional lateral deviation. This methodology differs from the more standard approach of path-planning/path-following, as there is no explicit path reference used; the control reference is a target acceleration vector which simultaneously induces changes in direction and speed. The lower level control distributes vehicle targets to the brake and steer actuators via a new and efficient method, the Modified Hamiltonian Algorithm (MHA). MHA balances CG acceleration targets with yaw moment tracking to preserve lateral stability. A nonlinear 7DOF two-track vehicle model confirms the overall validity of this novel methodology for collision avoidance.