And yes, you do have to pick and choose the highways you ride, and the time of year or time of day you ride them. In some places highways are busier during the week than on the weekend. In other places it is the opposite. In some places highways are busy during holiday times. In other places they are quiet during holiday times and busy during working/school times.
For example, there's a stretch of highway between Healesville and Narbethong, near where we live, called the Black Spur. It is one of the main highways out to the area where we live, and we would not advise cycling it ... especially not between about mid-December and early-February, when the holiday crowd are coming out from Melbourne to the country. And even through the rest of the year, really early in the morning, and really late in the evening would probably be the best times to attempt it. It is a very, very narrow, twisty road that climbs up and over a range ... 20-ish km of switchbacks and twists and turns, with no shoulder, and often lots of impatient traffic.
If you want to cycle northward out of Melbourne, there are other choices.
But you observe (when you're riding and driving) and figure it out. These days you can also get ahold of things like traffic density maps etc. to help you figure out when the best time to ride the highway might be ... or if the highway is worth riding at all.
This highway, below, is one that is near where we live in Australia. It can be quite busy during school holiday times (mid-December to early-February), when the Melbourne traffic comes up here (this is a somewhat touristy area because of the lakes and mountains around) ... so we don't ride it then. In fact, we hardly ride any of the highways in that area then. We go to less touristy areas to ride.
This highway can also be quite busy during "peak" hours during the week ... going to work time, going home time. But on a weekend in September (when the photo was taken), it can be a decent choice. The shoulder sort of comes and goes, but pick the right time, and the traffic density isn't too high, so it doesn't matter too much.