I got this from the accident report from the wreck at Monon. It gives the mesurments of the speed limit sign:
"A speed-limit sign, governing south-bound movements, is located west of the auxiliary track adjacent to the main track on the west. This sign is 30 feet west of the center-line of the main track and 1,103 feet north of the junction switch. It is 15 inches in height and 36 inches in length, is mounted on a mast and is 5 feet 6 inches above the level of the tops of the rails. It bears the numerals "15-15" in black on a yellow background."
Also if you have the Marquardt CD check out picture DSL000515003. If you look just to the left of the two tall poles on the right side of the picture you can see a speed limit sign.