Morning trains arriving in the city from the western suburbs are packed with passengers like tins of sardines. There's no room for more commuters. We need more rail capacity. The roads are full too but a road lane only accommodates 2000 cars, or 2400 people, per hour, while the same amount of space given over to a rail line accommodates about 20,000 commuters. If we improve public transport services in western Sydney, we will relieve road congestion too, because people will have another commuting option.

This is why regular public opinion surveys tell us most people — about 60 per cent to 70 per cent — think money being spent on roads should be spent on public transport instead. NSW Government plans to spend $11 billion on the WestConnex tollway – a tollway that widens the M4 and M5 and builds a six-lane tunnel between the two — doesn't jell with what most people want.

There's good science to back up the commonsense view. It goes like this: public transport operates to a fixed speed, a timetable. Most people will take whichever transport option is fastest. They don't care about the mode. If public transport is quicker they'll catch a train or a bus, freeing up road space. If driving is quicker, they'll jump in their car, adding to road congestion. In this way, public transport speeds determine road speeds. The upshot is that increasing public transport speeds is one of the best options available to governments and communities wanting to reduce road traffic congestion.

There is plenty of evidence to prove the point. When Sydney's train service reliability disintegrated in 2004 and the unusual decision was made to slow the city's rail network, embedding the slower speeds in the 2006 timetable, road speeds fell and congestion increased. Average road speeds were sitting on 34 km/hour before things went pear-shaped on the rail network. Afterwards, road speeds dropped to about 30 km/hour and have basically stayed there.

The new timetable has made rail travel quicker for many areas throughout Sydney but it is still too early to see if there have been improvements in parallel road speeds.

This relationship is one of the key mechanisms that make city systems tick. It is basic microeconomics, people shifting between two different options until there is no advantage in shifting and equilibrium is found. We can see this relationship in data sets that make comparisons between international cities. Cities with faster public transport speeds generally have faster road speeds.

With all this evidence behind us, it's hard to understand why anyone would want to build the WestConnex tollway.

The $11 billion earmarked to expand existing motorways that filled up with traffic not long after they were first opened, could be far better spent on public transport. A project like a second Sydney Harbour rail crossing is one of the things we won't see because of WestConnex. It would improve operations across the network by freeing up the bottleneck in the central business district that's stopping Sydney Trains from putting on more services across a whole suite of lines. If you free that strategically significant choke point, existing spare capacity on outer network links could be used to increase capacity across the entire network by a staggering 50 per cent to 60 per cent.

That's how you make big improvements and a genuine dent in the problem of road congestion and cut petrol bills for car commuters.

Dr Michelle Zeibots is a research principal at the University of Technology, Sydney's Institute for Sustainable Futures, specialising in the analysis of sustainable urban passenger transport systems.

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This story written and produced by the University of Technology, Sydney, for Brink, a publication distributed monthly in The Sydney Morning Herald