On my daily commute I encounter a couple of stretches of road where two lanes merge into one, in one instance after traffic lights and another between two mini roundabouts.
I am no amazing driver but find I can judge the speed of traffic in each lane and slot in nicely so nobody has to break or accelerate dramatically in either lane
But every single time there is someone who carries on at whatever speed until last minute and then has to slam the brakes in to fit in behind another vehicle who was always a bit in front in the other lane, or gets muddled and brakes anyway, and causes a small bottleneck
Why?!