The paper reports on a study commissioned by the Highways Agency to characterise, measure and model the delay costs associated with all types of abnormal load, supported by observation of fourteen movements, and monitoring of about sixty using MIDAS loop detector data. The study has also gathered statistics about load movements, observed industry practice, and consulted stakeholders drawn from the haulage industry, waterway authorities, utilities, police, and local and national governments, several of whom form an expert panel advising the Highways Agency on abnormal load issues including alternative water transport and environmental impacts.ghways Agency issues about 500 permits annually for the movement of large or heavy abnormal loads, and a larger number of authorisations of wide load movements. The British government pursues a water preference policy to minimise disruption. To make fair comparison of costs and impacts between road and water, various elements must be taken into account, the most important being the cost of road delays caused by the load; operational costs of haulage or shipping, removal of street furniture to allow passage, police escorting; intermodal (eg craneage); and other externality impacts such as accidents, noise, pollution and road damage. Delay to other traffic can result from vehicles slowing down when overtaking a load, or queuing behind it, or from complete road closures. While the majority of abnormal loads appear to cause little or no disruption, a few cause severe delays. Knowing the effect of day and time of travel is essential for the authorities when planning authorisations. Modelling of an abnormal load movement treats the journey as a sequence of road sections, each with its own data including length, ambient traffic volume, load speed and available capacity. Although queuing can be treated as deterministic, that is due to an excess of average demand over average capacity, it is complicated both by the forward movement of the load and the possibility of traffic overtaking. This blurs the distinction between a queue and a body of moving traffic, and can be handled properly only by a horizontal queue model which takes account of the speed and density of traffic. The approach relies on conservation of flow equations at the boundaries between different traffic regimes, together with assumptions about speed/flow relationships, though the results are not particularly sensitive to the latter. The resulting queue size can be expressed in conventional demand minus capacity form with certain modifications depending on the speeds and densities of traffic components. An example where a movement produced queuing which was observed both directly and through MIDAS detectors has been used to validate the queue model. From the hauliers' viewpoint, much planning effort is in gathering information about the state of the network. A new on-line portal called ESDAL, to be launched by the Highways Agency in Spring 2005, will eventually provide near-real time route planning assistance as well as a means of notifying authorities. Of concern to several stakeholders is the impact of Abnormal Loads on safety, pollution, climate change, infrastructure and the environment generally. While there is no direct evidence on these impacts, other data suggest that water transport has significantly less impact than road transport. Consequently there is interest in the possibility of further promoting water transport and improving facilities. On the other hand, the size of externality costs, when compared to those of operation, congestion and other disruption, may not be enough to affect decisions about whether to allow a load to be moved by road. The model which has been developed is being used to evaluate the optimum timing and route choices for high-profile Abnormal Load movements, and has the potential to be enhanced to take into account other factors besides congestion. For the covering abstract please see ITRD E135207.
Abstract