Vice-Chancellor's Seminar


Congestion - Friend or Foe?

By Emeritus Professor W. R. (Ross) Blunden

(formerly Foundation Professor of Traffic Engineering,

University of New South Wales)


  This paper distinguishes between congestion due to inefficiency and that due to the pressure of demand. The former kind needs to be attacked with vigour but the latter should be embraced as an ally and used to achieve a better utilisation of the modes and a more balanced distribution on the ground of interacting land-use activities.  

April 1983.
Or in the night, imagining some fear,
How easy is a bush supposed a bear!"
A Midsummer Night's Dream

The traffic engineer no less than his structural or electrical engineering counterpart needs his Hooke's or his Ohm's Law. Much of the early research carried out at the School of Traffic Engineering after it was established in the University of New South Wales in 1956 was concerned with the application of queuing theory to traffic flow phenomena and it was not long before an Ohm's Law of traffic emerged. It relates the time taken to traverse or pass through a traffic facility (a length of road, an intersection, a toll booth, a baggage counter, a cash register channel at a supermarket) to the level of the traffic flow or demand rate in a very characteristic way. Unlike its structural and electrical counterparts which give simple linear relationships between strain or voltage (the analogues of travel time) and the stress or current (the demand rate or flow of traffic) the relation between travel time and flow is highly non-linear. The travel time rises slowly at first but as it approaches the capacity of the traffic facility the increase is rapid and asymptotic to the capacity ordinate. This behaviour taken together with the fact that the demand slackens as the journey time increases ensures that a traffic system will settle down into an equilibrium state. The implications of this are profound and completely at variance with the traditional wisdom that has encouraged the transport planner to provide capacity pro-rata with an open-ended extrapolation of demand lest our cities suffer a thrombosis and collapse.

Now the foregoing dash of theoretical jargon is only presented here to allow me to assert in simple words that there are two kinds of congestion - one arising from inefficiency and a much more dramatic kind that arises from pressure of demand. The former is an enemy and must be confronted, the latter needs to be embraced as an ally. The first kind of congestion may be thought of as the "Main Street" problem that results from a bad mix of through and local traffic, inadequate pedestrian facilities, poor parking provision and controls. It is further manifested in inefficient traffic signals and intersection layouts, lack of right turn pockets, inappropriate frontage land use zoning, insufficient road/rail transfer terminals, intrusion of through traffic into local streets, poor signing and road marking and so on endlessly. It is fair to say however that the on-the-job professional traffic engineer has, over the past two decades, resolutely and successfully attacked many of these problems, to the extent that twice as much traffic gets around our towns and cities as well as half as much did twenty years ago.

When the traffic/transport engineer entered the urban planning lists his principal challenger was the other kind of congestion - the virtually unbounded demand for car travel. Initially he sought to overcome it by recommending more and more road capacity. In Sydney five 8-lane freeways were to converge on the city' s doorstep at Ultimo. Although the era of super-imposing massive rural type freeways on established urban land use and transport structure has passed, the transport planner is still ready to prescribe a major bridge or a clover-leaf interchange to solve a bottleneck problem. In some cases such solutions may be justifiable but not generally. Just such a proposal has recently been the subject of the Warringah Transport Corridor Inquiry - but that is sub-judice right now. We can however use another currently debated problem to gain insight into the mechanism of congestion and its effect on the land-use infrastructure of a city. It is the question of a second Harbour Bridge in Sydney.

The existing bridge is one of the great traffic concourses of the Nation and has the capacity to handle road traffic at the rate of 15,000 per hour. But if only 500 vehicles were to approach its great deck between 8 a.m., say, and one minute past there would be acute traffic congestion and a prima facie case for duplicating the facility there and then. A demand intensity of 500 vehicles in one minute produces a flow of 30,000 per hour and the bridge can only handle them at half this rate. What, of course, the hapless 500 do is to spread themselves out over two minutes and all is well. The spreading goes on up to two hours even in big cities, and in the case of the Sydney Bridge this, together with a modest occupancy rate of say 1.25 persons per car, allows nearly 40,000 persons to move between north and south. One particularly important point to stress is that any individual traveller suffers no more delay than that corresponding to the critical speed (some 30-40 km/hr) of operating the facility at its full load. He is not delayed or congested for the whole period of the peak as popular opinion tends to surmise. The spreading of the demand does call for some programming of the trips and there are perceived disadvantages associated with this. However a great deal of the spreading occurs naturally, due to the spatial distribution of destinations beyond the bottleneck and when one takes account of staggered working hours and flexitime arrangements the Shakespearian bear may well be imaginary.

But there is more to consider. As the peak stretches, public transport alternatives (if any) become more attractive. In the case of the Sydney Harbour Bridge there is capacity for a further 30,000 person trips per hour in just one direction of movement. But more importantly, when all transport capacity is used to its "tolerable" upper limit people are influenced to change the pattern of demand. Committed individual desire lines may not change much but new developments and growth patterns may change dramatically. The Sydney Harbour Bridge illustration can be expanded to tell something of its own life story. When it was opened just over half a century ago, Sydney's North Shore was a collection of "villages" around the railway stations and the Warringah Peninsula was a holiday resort. Today North Sydney is a city the size of Adelaide and the Peninsula has jobs within its boundaries for about half of its 60,000 workers and many of the 30,000 external jobs its workers need are available on the north side. The North Shore has major market and commercial centres at Chatswood and Hornsby and a university, the growth of which is undoubtedly encouraged by its relatively good accessibility with respect of its competitors on the south side.

Just over a year ago when the Jubilee of its opening was being celebrated one National Daily pictured it under full load with the headline "A City Facing Strangulation". Might I be so egocentric as to recall that I styled an article published about the same time in the University of New South Wales Quarterly - "The Bridge. Sydney's Master Traffic Controller".

Now I am conscious that the Sydney Harbour Bridge is a long way from Wollongong and I apologise for making it the main theme of my narrative so far. But let me say that this glamour example of the bottleneck problem is not a one-off situation in just one city. Bottlenecks, big and small are of wide spread occurrence in all our cities and they hold the key to the philosophy that large cities should become collections of sub-cities. Such a philosophy should not of course become a doctrine but rather a guiding principle. In the world of real cities topography, history and the existing land-use transport infrastructure call for a great deal of variety in the solutions we devise. Nevertheless we can go about the job of providing for their future with greater confidence if we understand that the strong feedback forces embodied in their traffic allow one to trim the ship without fear of it capsizing. In the discussion so far the feedback mechanism that established equilibrium is, if we relate it to physics, of the negative kind. But feedback may also be positive and this induces rapid growth. This is of course evident in traffic systems and my reference earlier to the growth of Sydney's North Shore after the opening of the bridge is a spectacular example of this. In a somewhat ironical way it is the acceptability of the growth effects of new traffic facilities that has made us reluctant to accept the inevitable tapering off when negative feedback forces take over. This is understandable but the consequence that is often missed is that when a given level of growth takes place the activities concerned go "critical" and further growth can go on in a self sustaining way. Planning should recognise such possibilities and encourage them. Other examples of functionally specific activities that confine the transport task are to be found in the area zoning of schools and the decentralization of shopping centres.

With this background we can now address the general theme of this Symposium. In the sense that transport is the apparent begetter of congestion it is indeed a tyrant. But we have seen that congestion occurs because we all want to get to the same place at the same time by the same mode. Congestion is the inevitable consequence and without full understanding of the process we blame transport. We expect far too much of our transport especially our motor cars. This is due in no small measure to the fact that we spend so much money on them. We also use them inefficiently. What transport organization could survive if it ran its fleet at 20% load factor for something less than 10% of the year? And what of the house-wife who pays $500 per tonne-kilometre to transport home the kilogram of butter for which she made a special trip to the corner shop. The companion theme of the Tyranny of Distance is also worthy of interpretation in terms of the foregoing exposition. Here again distance tends to be viewed as being synonymous with transport when in fact it is directly related to the spatial distribution of land-use activities and here again there is a dominant aspect - the spatial relationship between our homes and our workplaces. Long commuting distances can be traced back to an earlier transport era - that of the high capacity commuter railroad which encouraged the concentration of jobs in a central area and permitted the working population to ebb and flow for long distances to the residential suburbs. The motor car demands a radically different distribution of land-use and the jobs should be dispersed amongst the workers. Much of our recent development has responded to this. As a result we have two radically different partners endeavouring to make a good job of their marriage, and by and large they are succeeding. If one looks to the future the portent is encouraging so far as the task of reducing our commuting travel is concerned. With the steady reduction in manufacturing jobs, future job opportunities will occur more and more in the tertiary and service categories which are more easily located closer to home. If one takes into account the revolutionary impact of electronic "transport" we may in the future not need to leave our homes for work purposes. Trends such as those envisaged will undoubtedly reduce work travel but will no doubt increase recreational travel - we may even spread the traffic peaks of the future over 24 hours!

With this tempting prospect in mind I should quickly bow out for our ally will then have become our friend and transport our servant - a means to an end not an end in itself.

This perspective was perceptively grasped and succinctly expressed more than two hundred years ago by the great scholar and philosopher Dr Samuel Johnson. Late in his life he was travelling back to London after a visit to his birth-place, Litchfield, accompanied by Boswell who recorded in his celebrated biography the following:

I enjoyed the luxury of our approach to London ..... of being whirled along with such a companion and said to him "Sir, you observed one day ... that a man is never happy for the present but when he is drunk. Will you not add - or when driving rapidly in a post-chaise?" Johnson, in characteristic vein, replied "No Sir, you are driving rapidly from something or to something."