EDIT: This post became waaay longer than intended so you've been warned lol.
I've been driving trucks for almost a decade now so I'm going to give my honest take on the sliproad/general traffic merging element (which will likely get me flamed by some lol).
Even though this seems like a pretty obvious statement as most people realise that trucks are big, slow, and need a lot of space to merge, I think it's hard to really grasp the full extent of it until you've driven one yourself.
Even changing lanes is much harder than it looks as an EU truck can be up to 60ft in length and when looking in the comparatively small mirror it can be really hard to tell whether a car is actually behind you or alongside unless there's a fairly sizeable distance - this is only exacerbated by the dark and adverse weather conditions (rain, fog, etc).
It's clear that a lot of drivers aren't aware of this because almost daily I'll indicate to move over and the car behind me will hold back to let me go, giving what they assume is enough distance to make this obvious, but in reality I still can't tell whether I'm actually clear of them until they flash me in many cases.
If the car doesn't flash it's hard to tell whether they're letting you in or instead just driving alongside you but just maintaining their speed rather than easing off to let you in - they may not have even seen that you're indicating if they're just forward of your tail lights.
I've also had a fair few close calls where I've held back for a few seconds just to be sure they're letting me in, and they've then got impatient and floored it/undertaken me just as I've started moving over, causing me to have to swerve back into my original lane.
It's much easier now I have a rear facing camera but it's also staggering how it's so clear from the camera that they're holding back, yet often impossible to tell this when you compare via the mirrors by looking back and forth.
Regarding merging/changing lanes, you absolutely have to drive 'assertively' in some instances - not the same as cutting people up btw. The reality is that a lot of the time you're just not going to have the necessary space to move over and get up to speed without causing people to slow down, and you can't stay in the same lane forever.
People don't tend to leave say 6-8 car lengths between them and the vehicle in front, and if they did another car would use the opportunity to nip into that space from the inside lane. Also, it can take a whole mile to get back up to spees after slowing down and people often don't like to get stuck behind lorries so won't let you in.
So what you often have to do is make a calculated move, giving the cars behind you sufficient space to adjust their speed without braking excessively/dangerously. But make no mistake, you will absolutely have to make cars slow down in many cases. It's when truck drivers get this wrong that it becomes dangerous.
It's the same when we're pulling out of a side road. You will often see a lorry pulling out in the distance, and despite the fact it's a long way ahead you'll still end up slowing down as it won't have got up to speed by the time you reach it. It will have absolutely caused you to adjust your speed but it was so far away when it pulled out that you barely registered the fact that you instinctively slowed down as it was so gradual.
Now the issues arise when drivers mess this up. Truckers tend to work long hours (50-60 a week in general) so when you're 11 hours into your fourth shift of the week you may be a bit frazzled. They also tend to do irregular hours with last minute changes. For example, I was meant to load at 08:00 this morning at a client's depot 30 mins from my house but they messed up the schedule.
As the client's planner didn't have my number he emailed me at 17:30, but I didn't pick it up - the system was still showing the original loading time. I didn't even know my boss had given them my personal email tbh (I usually only check my gmail before bed as we never use email at work and always call each other via hands free). I had no reason to expect anything to change.
At 8pm they finally thought to email my boss but he didn't read it till 21:25 as he was out with his family. So I get a call at half nine telling me I'm actually loading at a depot 90 mins away at 07:30. So now I need to suddenly be up two hours earlier as I'll now need to fuel up before leaving and also have to open up the plant when collecting the truck as nobody will be there that early.
So suddenly I've got to be up in seven hours rather than nine and I'm only just leaving the gym and still need to get home, shower and get my stuff ready etc. I could say no in theory but this will cause big problems so not really an option - if I miss the concrete pour the end client can bill our client for loss of earnings for his whole team sitting around unable to work due to the materials not being delivered/contractual terms not fulfilled. Thousands of pounds on the client's head due to me wanting 2-3 more hours in bed. Shit but it's the reality.
So after already having had a crap night's sleep the night before and then smashing the gym thinking I was going to get 7-8 hours in bed, I had to get by on five hours sleep and do an 11.5 hour shift in an unfamiliar town today.
So sometimes drivers will be tired and this no doubt contributes to them misjudging the occasional manoeuvre, or even just the law of averages whereby they make a momentary mistake on one of the many 50 hour weeks they spend on the road. You can say they shouldn't as professional drivers but in reality they do.
Alternatively, the car driver behind them may just not be paying due attention (listening to music/radio, on hands free call, preoccupied with thoughts, etc) and doesn't notice early enough. Maybe instead of 'oh, that lorry up ahead's pulling over, I'd better slow down' it becomes 'tra la la la....oh shit shit shit!'.
These are just my thoughts as somebody who's spent tens of thousands of hours driving trucks and has trained dozens of new drivers.