On Fri, 31 Mar 2017 15:51:29 +0100, Roger Mills wrote:

No. It doesn't need much of a gradient to slow them down. A heavy
lorry's drag to weight ratio and rolling resistance to weight ratio are
quite low. But add to that a component of the weight as a result of the
slope, and the required tractive effort increases dramatically.

20, 30 years ago you'd often come across a lorry doing 30 or 40 mph
up a motorway "hill". These days the vast majority of trucks have
enough power to maintain their speed on the relatively gentle
inclines found on motorways.

The major problem is the slight differences in the calibration of the
limiters (*) meaning a truck attempts to overtake with only 1/4 mph
speed advantage. Happens on the flat, not just hills, thinking of the
dual carriage way parts of the A1 south of Newcastle and the M6 south
of Lancaster.

(*) I have sneaky feeling that modern "limiters" are really rather
sophisticated cruise controls that also have control of the brakes.
Often see a truck trundling down a hill at an apparent steady speed
but it'll briefly brake, maybe only once or twice but well before a
human would notice and far shorter.

--
Cheers
Dave.