Downhill continuous braking. Please help

I'd have to say I can't get the math to work in a way that makes those four test alternatives equivalent. Perhaps there is some empirical evidence re the ability of typical brakes to dissipate heat that makes the math work out differently than expected?

Thanks Blink, it has me copletely baffled at present.

It's easy...the peron who wrote the spec was not a mathematician. You were not expected to analyze the math. Just do the test.

May be they are taking into account lower level of heat generated during braking at lower speed & possibly better heat dissipation of the brake drums cooling down over a greater time period.

the increased lengths consider the heat dissipating ability of the brakes during the longer time at slower speeds. Whit is important is to demonstrate the braking system can reliable dissipate the kinetic AND potential energy involved in stopping the vehicle at its maximum weight. These US truck/trailer braking requirements are really far easier to meet with new components and are significantly lower than what real world requirements should be. They SHOULD be conducted using aged components at 'minimum allowable lining and drum thickness' to firmly establish minimum acceptable performance.

You should read what the FAA requires of aircraft braking systems.