Many readers will have seen this now-ubiquitous chart before – it’s from Melbourne 2030 and has been republished countless times. It shows the proportion of metropolitan jobs accessible within 40 minutes travelling time from different parts of Melbourne by car and by public transport.

I’ve never been happy with this chart because its simplicity is deceptive – I don’t accept the implicit premise that public transport should be judged on the same basis as cars. I’ll come to that shortly, but first there are some technical shortcomings that need to be addressed.

One is that the chart doesn’t say how the data range intervals are determined – are they equal counts? Are they based on a ‘natural break’ in the way the data is distributed? It’s not possible to be confident that they portray the situation with either public transport or cars in as objective a way as possible.

Another shortcoming is that a mere three categories is very limiting. If you live in Sunbury (say), the chart says you can drive to between 3% and 25% of metropolitan jobs within 40 minutes. That’s an enormous range – a factor of more than eight between the lowest and highest values. It’s essentially a useless piece of information. And the maps give a misleading impression of how many Melburnians live in areas with the poorest accessibility. There are very large areas on the fringe that have a tiny population e.g. there is a 9 km wide greenbelt between Melton and Caroline Springs. Much of the outer north east is a catchment area.

Yet even with these technical flaws, there is some intriguing information. For example, the majority of the population can access no more than 2% of metropolitan jobs within 40 minutes travel by public transport.

But is that necessarily as bad as it sounds? Two percent of all employment in Melbourne is a bit over 30,000 jobs. More would be better, but that’s still a lot of jobs – the entire City of Greater Bendigo, for example, has around 40,000 jobs (population circa 100,000).

Conversely, some residents can access 26% or more of the metropolitan area’s jobs. That’s at least 400,000 jobs. I think the significance of this can be overstated – diminishing returns set in well below this figure. After all, there are only so many jobs for barristers or baristas in any city.

But my main beef is the comparison the chart sets up between cars and public transport. The implication is that public transport is deficient because, for example, it only offers Sunbury residents access, at best, to 2% of Melbourne’s jobs (within 40 minutes) whereas cars offer them access to as many as 25% within the same time frame.

This sort of thinking sets public transport up for a fall.

For a start, it doesn’t allow for the fact that a particular form of public transport – trains – favours long trips. One reason the median journey to work by public transport in Melbourne is nearly double that of cars – 55 minutes vs 30 minutes – is that trains enable some workers to trade off a long commute to the CBD (which incidentally has around 15% of Melbourne’s jobs) for a spacious residence in the suburbs.

A resident of outer suburban Werribee, for example, can catch the 7.45 am service and 37 minutes later arrive at Flinders Street at 8.22 am, after stopping at Hoppers Crossing, Aircraft, Laverton, Newport, Footscray, North Melbourne and Southern Cross stations. The 8.01 am train from far-flung Berwick takes 47 minutes to get to Parliament (at 8.48 am), after stopping at ten stations. Of course the walk and wait times have to be added to these in-train times (and they push the trips over the assumed 40 minute threshold), but the trade-off suits some people.

But most importantly, the chart ignores the fact that trains, trams and buses are inherently different from cars. Passengers walk or drive to a stop, wait for a service and walk to their destination at the other end. Most services stop regularly to take on and let off passengers and some travellers have to change to another service during their journey. Passengers frequently have to stand.

The time and comfort penalty can be minimised, at a cost, by a denser network of services and higher frequencies, but at the end of the day trains, trams and buses are not cars. In the vast majority of cases public transport still won’t offer the on-demand convenience and directness of a car.

Public transport is by definition a different beast to a car. In the AM peak it typically has to carry many passengers from multiple origins and in the PM peak it carries them to multiple destinations. Public transport is about sharing – that’s why it’s more sustainable than car travel. It only seriously out-competes the car for mode share when the car’s inherent advantages are neutered by factors like congestion – the key example in Melbourne is commutes to the  CBD, but they’re only a small proportion of all travel in the metropolitan area.

None of this means that public transport in Melbourne doesn’t need improving. It’s just that it shouldn’t be expected to deliver on the same terms as the car. That would be a bit like expecting students to learn as well in a class of 20 as they would with individual tuition. Or comparing the comforts of a dormitory with those of a private hotel room.