The Impact of Abnormal Loads on Road Traffic Congestion

The Impact of Abnormal Loads on Road Traffic Congestion


N Taylor, TRL, UK


Large slow abnormal loads have been monitored and a model developed to estimate delays they cause to other road traffic. The model assists decisions about when to allow loads to travel and whether alternative transport by water should be considered.


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.

The English Highways 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. Typical of the largest loads are electricity transformers, metal castings, bridge sections, reaction vessels, wind-generator parts, dump trucks, and some specialist items such as boats and aircraft components. The British government pursues a Water Preference policy to minimise disruption. Water transport is used or considered for some types of load, especially where water access to facilities is straightforward. However, the absence of long-distance internal waterways like the Rhein and Garonne means that most abnormal loads still move by road. 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. Delays can be highly variable, and are found to be particularly sensitive to certain inputs: the speed of the load, the number of lanes it occupies, and the day and time of travel. The heaviest types of load (over 150 tonnes) are limited to about 19 km/h, and may average as little as 8 km/h. These also tend to occupy two standard lanes, so representing a considerable obstruction to traffic even on 3-lane motorways. Other types of vehicle, carrying lighter and narrower loads, can achieve speeds of 40-80 km/h, and depending on their shape, some loads up to about 4.3m wide can be confined to one lane for much of the time. Measurements indicate that the capacity of lanes for passing traffic is substantially less than their normal free-flow capacity, possibly as low as half but probably nearer two thirds, although individual figures are highly variable. Due to the varied location of engineering facilities and destinations, and also weight and height restrictions on some major roads, convoys often need to use single carriageway roads. Police may then implement rolling closures, and diversions if suitable alternative routes exist. Routes and conditions are highly specific to each move. 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. In addition to the need to avoid peak period traffic, local authorities may impose constraints on daytime movement in urban areas, and police on nighttime movement on unlit roads, and there are further constraints on access to port or other facilities, drivers? hours, and rest areas. To predict delays correctly, it is necessary to know the time of arrival on each road section and the likely volume of traffic there. In the UK, detailed hourly traffic profiles are available for many motorways and trunk roads. For lesser roads, it is necessary to apply general profiles to average daily traffic counts.

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.


Association for European Transport