On Wednesday, April 2, 2014 3:55:22 PM UTC-4, Graeme Wall wrote:
As anecdata, I don't recall seeing an open platform bus on the motorway
with passengers on board.

My father once told me that the first time he drove on the 285 to Heathrow, he took a wrong turning near the airport and found himself on the M4 and had to go up to the next junction and come back. This would have been in the late 60s/early 70s and Wikipedia says it switched from Routemasters to Swifts in 1971. I think it would have been more likely my dad was driving the latter.
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Roy