Alright. Even if we eventually figure out what "broad-jumper's pants" were, will we ever understand the concept of a "motor robe?"
Described as "the finishing touch of luxury," this advertisement for "Robes by Chase" (made by the L. C. Chase & Company of Boston) tells you everything you'd want to know about the available colours, weaving techniques, and superior fabrics of a motor robe...but it doesn't tell you what they're USED for.
I initially thought they were just a type of upholstery, but check out the picture: the robe is hung on a rod that's attached to the front seat, conveniently available for the driver to reach back and fondle now and then.
Was it used as a blanket during the winter? Was it a cushion in case of a car accident? The advertisement says that the motor robe "feels welcome, indeed, to silk stockings," maybe implying that stocking-wearers could put it on the seat under them, to protect their hosiery from rough seat fabric (though that doesn't explain why the women in the picture above are STARING at the thing instead of SITTING on it).
The motor robe...another of life's great mysteries.
(The New Yorker, February 16, 1929, page 48)
PS: It appears that the motor robe was patented in 1925. The only other online reference I can find refers to it as a "lap robe," which means it probably WAS something meant to keep passengers' legs warm.