The argument for more/bigger roads (particularly motorways) is often that tradies and others need to carry their tools and equipment to jobs, so they can’t use public transport.
Perhaps that’s true, but they are the minority of people on the road.
According to a 2012 ABS study, only about 7% of people avoid using public transport (for work or study) because they need to carry tools etc. Another 10% say they have to take their own vehicle.
The more significant reasons are that no public transport is provided (about 30%), they want convenience/comfort/privacy (28%), the services aren’t convenient (25%), or the travel time is too long (18%).
So for the vast majority of people on the roads, it’s because the public transport service isn’t good enough, not because they inherently can’t use public transport for those trips. There might be some overlap in responses of course, but I suspect most people travel most of the time with nothing they can’t carry themselves.
And in fact if you jump on public transport at the right time, such as early morning and early afternoon, you’ll see plenty of blue collar workers using it — presumably those who don’t use personal tools, or are able to keep them securely on-site.
Of course, if building new motorways were really to prioritise those vehicles that have to use them — trucks carrying freight, tool-carrying tradies and so on — there’d be priority lanes to make sure those vehicles got through without getting caught up in congestion caused by individuals in cars. But no roads have such priority lanes.
Edit 29/4/2017: Added top image of a tradie and his tools on a train to Pakenham.