From my own reading, it seems that the traffic congestion was a combination of the relatively undeveloped state of the main roads in the area (mostly single carriageway and quite narrow) and the habit at the time that many families had of taking a drive on a Sunday.
Reading this thread has been very interesting, because despite considerable coverage of the G-ARPI crash in various books I had never before realised either the situation regarding suspicion of the ground engineering staff's actions or indeed that the low hours that Jeremy Keighley possessed on type were at all commonplace at the time.
My own first flight was on a Trident 3B in 1977, as a consequence I have read up on both the Staines and Zagreb incidents and as much else about the Trident as I can.
Did the droop lever on the early mark Tridents ever have a speed guard fitted? I know that there had been several incidents previous to G-ARPI's demise, at least one of which was described as "We just about managed to keep flying until droops were reselected!".