Having observed, what, maybe two or three hundred drivers of commercial diesel vehicles, and verbally surveying, what, maybe 15% of those, I've come to the firm

Having observed, what, maybe two or three hundred drivers of commercial diesel vehicles, and verbally surveying, what, maybe 15% of those, I've come to the firm conclusion the overwhelming majority of them leave their engines idle due to a historic quirk, whereby it was widely considered to improve longevity of the engine to reduce the number stop-start cycles. This has been pretty much obsoleted by improvements in metallurgy, machining tolerances, and lubricant improvements - thermal cycling is still important, but that's a negligible concern over a 20 to 40 minute period in temperate climates. Modern turbo diesel engine cold-starts should be considered ready to drive in a gentle manner immediately, avoid full boost till operating temperature - but full boost should rarely be needed in most driving conditions anyways.