I travelled that road many times and it is very very narrow , built before LHR existed to connect the villages of Stanwell and Bedfont and is no longer fit for purpose with articulated HGVs and tankers . It has a river on one side and generally commercial properties on the other. The junction with Long Lane where the accident seems to have occurred is of similar vintage . A car headed north and then east has limited vision for oncoming traffic depending on foliage etc and as the truck ended up in the river it is all too likely that momentary inattention or just blocked vision saw the car pull out into the ah of the truck who served and ended up in the river but destroying the car too.
This road should have been upgraded decades ago but it is on the boundary of a London borough and Surrey CC and along way from their head offices so nothing ever gets done.
I dont mean to over speculate one the accident except for the fact I nearly lost a friend there decades ago, in his case the truck pulled out and he swerved into the river well facing down its bank shaken but unharmed this time three young people lost their lives tragic indeed