ARCADY Health Warning: Account for unequal lane usage or risk damaging the Public Purse!

摘要

The ARCADY computer program was first released in 1981. Developed by the Transport and Road Research Laboratory, the program predicts capacities, queues and delays on roundabout approaches from user-specified geometric and origin-destination data. The latest release, Visual ARCADY/4, was released in October 1996. The program development can be studied by acquiring and reading references 1 to 10. Over the years much development work has been carried out with many new features added to the program — for example, geometric delay, formulae for grade-separated entry arms, pedestrian crossings on approaches, accident rate and type predictions, and, most recently, the addition of a more user-friendly data entry module. However, in the intervening 16 years, the 'heart' of the ARCADY model has remained essentially unchanged, i.e. the use of an empirical formula to derive, using geometric and traffic demand input data, the entry capacity of each arm as a function of the circulating flow across the arm entry. This model is well-proven. However, of itself, it can take no account of either unused or unequally used lanes or flared sections on the entry approaches (see photograph). ARCADY is, in fact, completely 'blind' to such occurrences, and as a consequence may produce hopelessly over-optimistic predictions. In the case of unspotted erroneous Traffic Impact Assessment (TIA) submissions, this can lead to (and probably already has) the construction of inadequate designs that have to be 'improved' and/or replaced at a later date, often at a Highway Authority's considerable expense! Following the author's public voicing of these concerns in December 1996, the Transport Research Laboratory (TRL) has agreed to place a warning in their new release, Visual ARCADY/4 documentation. They have also invited the author to submit a report illustrating the problem and any proposed solution methodology, this for their consideration and possible inclusion in future ARCADY User Manual documentation. This paper basically constitutes such a report. Examples illustrating the inability of ARCADY to account directly for non or unequal lane usage are given, together with details of the author's modelling methodology aimed at achieving more 'realistic' queue and delay predictions.