I have found that courtesy can be contagious, I always try to let people into traffic (except BMW drivers who will travel at some speed down the outside of queuing traffic, ignoring all the 'lane closed ahead in 400, 200, 100 metres signs' until they arrive at the pinch point and proceed to try and bully their way in, all this while other sensible drivers have been merging in ahead of time - but you have to understand that BMW drivers do consider that their time is more valuable than anyone elses).
I have often let someone in and watched as they do the same to some other driver further along the road. I also make a point of moving to centre lane of motorway when approaching junctions to give joining traffic a bit of space. I have been driven ( no way would I drive) in Cities like Bangkok, Jakarta and Manila where they make a 3 lane road into a 6 lane one no problem, they are forever tooting their horns, but not in anger, just to say 'i am here', it all seems so tangled up but seems to work out (except when they have the dreaded 6 hour traffic jams, drivers in these cities have a wide necked bottle in their car as standard equipment, don't know what the women use LOL )