Recent debate on the letters page of The Canberra Times has canvassed the history and value of ideas in Canberra's planning, such as the separate town centres, or the Y Plan.

In fact there have been four distinct planning concepts or pressures that have influenced Canberra's planning: separate towns, linear city, single node, and dispersed development.

Walter Burley Griffin envisaged Canberra developing in a way no longer feasible: of medium density apartments served by trams, like his home town of Chicago.

Development in the 40 years to the founding of the National Capital Development Commission in 1958 actually took the form of low-density cottages with gardens, in a few scattered suburbs, in the very small city being built around the grand axes, open spaces, future lake and bush-covered hills of Griffin's plan.

Although this early suburban development has been attributed to the influence of Ebenezer Howard's Garden City idea from England, in fact, it more reflects the growth of early suburbia in Australia, where Australia led the world from the late 19th century in the provision of detached cottages with small gardens.

The ''natural'' development of cities, in the absence of planning, is for a single node to come to monopolise high-density and prestige uses, such as government, offices and high-rise apartments.

At the same time, the rise of the car has meant that, without planning, all other uses tend to spread in an unstructured, low-density way across the metropolitan area. This reinforces car travel, results in a pattern of very long and dispersed journeys to work, makes public transport mostly unviable, and leads to heavy pressure to provide immense parking space in the single nodal centre. Many suburbs develop a long way from any but the most local services.

To overcome these problems, NCDC devised two approaches.

One was separate town centres in separate suburban regions. The idea was to provide major town centres on a par with Civic, so that journeys to work, and for services, could be minimised for those who lived in the catchment area of each centre.

However, as many have noted, once people began changing jobs, or several household members worked in different centres, the minimised journey to work objective was greatly weakened.

Nevertheless, the idea of major town centres in competition with Civic still has much justification, by reducing single-node dominance. It means that low-income families forced to the outer fringe don't have to rely on Civic for almost everything.

The second approach was the Y Plan, the idea of a single, central, public transport spine connecting all the town centres.

It was modified to a ''Y'' because the Gungahlin and Belconnen centres cannot be fitted onto one line. The concept underlying the Y Plan is that of the linear city.

According to this concept, in an ideal small city all major higher-density destination points for journeys from residential areas (offices, services, centres of government, retail, entertainment, etc) are located in a dispersed, balanced way along one public transport line.

Nearer the line is high and medium density housing, further from it, is lower density housing. At points along the line is paid parking. Such an ideal means balanced demand along the line and high-frequency, low-fare, all-hours, high-quality public transport. Car travel is lessened because it is convenient to just drive to the nearest point on the line.

The real-world application of this model to Canberra in the Y Plan has been hindered by the low density of Canberra, reflected in both distance of town centres from one another, and the low density of housing in each suburb. The extremely low density of Canberra was an unfortunate outcome of NCDC's desire for ''ideal'' planned suburbs, reliance on the car, and the precedence given to open spaces.

The Y Plan is also undermined by pressures for single-node (Civic) dominance, and by pre-existing developments that cannot be fitted to the line such as Queanbeyan, Fyshwick, and Weston Creek.

This amorphous, dispersed development pattern, which NCDC attempted to curb, is now continuing with developments such as non-airport uses at the airport, the new Molonglo valley districts, the expansion of Queanbeyan, and so on.

The Y Plan is not helped by the fact that main employment centres in central Canberra are not on one line and are separated by huge areas of lake and greenery, according to Griffin's plan.

There seems no option but to muddle through with these four different approaches, trying to maximise workability in a pragmatic way. Broadly, ACT governments should continue to support the idea of strong town centres and a viable public transport spine, and try to moderate the inevitable pressures for an over-dominant Civic and for unstructured, dispersed development.

There are two other things that should be done to make Canberra as livable as possible. One is to increase densities in all developed areas: for instance, there is an incredible amount of unused land in virtually every part of established Canberra which could be used for housing and other uses, without infringing the two main features of bush-covered hills and the best parts of the landscaping around Lake Burley Griffin.

The second is to encourage through incentives, regulation and infrastructure walking, cycling, motor scooters, light motorbikes, and small energy-efficient cars.

Paul Pollard is a Canberra town planner.