Using government statistics, he estimated that the average road freight delivery in the UK was delayed by 13 minutes during the morning peak period, 5 minutes during the day and 11 minutes during the evening rush hour. He argued that if these average delays were regular and predictable companies could simply build additional slack into their delivery schedules to accommodate them. This would carry a cost penalty as more vehicles and drivers would be required to deliver the same quantity of goods within specified order lead times. It would also increase the total amount of inventory in the road transport system at any given time. Professor McKinnon's calculations suggested, however, that the cost of financing this additional in-transit inventory would be a very small element in congestion cost calculation.

The main problem with traffic congestion is that it is not always regular. As the road network approaches saturation level, the traffic flow becomes unstable and more susceptible to the disruptive effects of traffic accidents, bad weather and road works. Journey times do not simply lengthen; they also become more variable. The probability of deliveries arriving late then increases.

Professor McKinnon explored the indirect effects of congestion experienced by factories, warehouses and shops when deliveries are delayed. Most calculations of the cost of congestion to the economy exclude these 'consequential' costs. This is partly because they are very difficult to estimate. Industrial and commercial operations vary enormously in their sensitivity to congestion delays. In the case of warehousing, for example, this sensitivity depends on the level of cross-docking, internal process times, dependence on vehicle pre-loading, the stringency of booking-in times, geographical location, the degree of just-in-time replenishment and the scheduling of deliveries over the 24 hour cycle.

The government-sponsored transport KPI survey which the Logistics Research Centre had organised in the food sector in May 2002 had shown that traffic congestion was only one of many causes of unreliability in the supply chain. Of the 15,252 lorry journeys monitored over a 48 hour period, approximately 29% were subject to a delay. Traffic congestion was the main cause of just under a third of these delays. Most of the delays occurred at collection and delivery points, where vehicles had to wait longer than anticipated to be loaded or unloaded. 'It is not only the road network that is congested' Professor McKinnon explained, 'it is also the reception bays of factories, warehouses and shops.' He argued that traffic congestion is being superimposed on a high level of schedule variability which already exists in companies' logistical systems and this amplifies its negative effect.

Professor McKinnon concluded his address by examining ways in which companies could reduce their exposure to mounting traffic congestion. He noted how the proportion of lorry-kms run between 8pm and 6am had increased from 8.5% back in 1985 to around 20% today as companies tried to avoid increasing day-time congestion. The extension of the Working Time Directive to mobile workers in March 2005, particularly its restrictions on night-time working, may arrest or even reverse this trend.

Increasing congestion on the road network is likely to encourage a modal switch to rail. From a broader transport policy standpoint, however, it might be preferable to encourage people rather than freight to transfer to rail. Professor McKinnon compared the extent to which a typical intermodal freight train and an inter-city passenger express could release space on the road network. Weighting a lorry as being equivalent to 2.5 cars and assuming an average car occupancy of 1.5, the passenger train could release around 3.6 times more road capacity than the freight train.

Cars, after all, are by far the main cause of traffic congestion. They are forecast to account for 70% of traffic growth between 2000 and 2010 as opposed to 4% for trucks, even after trucks have been assigned the 2.5 car weighting. Freight transport and logistics are therefore likely to be more the victims of traffic congestion than its cause.

A copy of Professor McKinnon's presentation can be found at the Logistics Research Centre Web site: http://www.sml.hw.ac.uk/logistics