Fizzie let's say you're going on a 20-mile drive in an estate car. The first mile is on a nice back road, a few lorries but not many, mainly cars about the size of yours. But to get where you really want to go you have a choice. You can do about 30 miles on similar roads to this one but it will mean stopping and starting a lot and therefore getting through a lot of fuel, which is expensive and it will take a long time. There will be lorries but not that many of them. Sometimes the drivers in the lorries won't see you and you'll have to brake to avoid them. You know that sometimes the lorries are driven right over the top of cars like yours and the drivers aren't prosecuted for doing this so aren't very careful about not doing it. So you want to avoid lorries but you don't really want to add 50% onto the length of your journey and probably around 100% to the time it takes.
So you opt for the 20 mile route which involves A roads with a lot more lorries. However, despite your lorry fear you know if you are assertive with your road positioning you can hold them off and make yourself more visible. Your car is painted in luminous orange so in theory they should be able to see you anyway, especially since you often have your lights on in daylight. Sometimes lorry drivers will yell things at you, but you've got used to it and besides, who wants to drive around on their own in a great big lorry, when they could just use a more economical car?
On the 20-mile route, there is a 1-mile stretch of back road from which lorries are banned. It's quiet, because there are only other cars on it but to get to it you have to detour across about 8 lanes of lorries. It sort of goes in the right direction but not quite and there is only 1 mile of it. You can just about pass other cars coming the opposite way. If there is a slow car in front of you (and the road often attracts slow drivers) you have to wait a long time to get past them safely. Then to get back on your chosen, more direct route, you have to cross a junction with a lot of lorries, many of which are routinely driven through red lights, at you.
Are you going to use the 1 mile of quiet, car-only road or not? It will give you a breather, but it's slow, out of the way, and so risky to get onto it it actually ups the danger quotient overall.
This is basically the decision cyclists have to make about cycle paths. Even the ones which look bloody marvelous from a car are rarely worth using if you are cycling fast and trying to get somewhere as they are just too disjointed. Cyclists have a right to use the road. Oh, and if you think cyclists are the ones causing congestion, you're thick. There's no other way to put that.