I am using CSX Speedboards (50 Mph, 35 Mph & 15 Mph ) and they get "clipped" by passing trainz, but that doesn't bother me.
Also with speedboards, in alot of cases, train will not obey a speedboard ahead until its last car passes, so you have to allow ample room for train length, for the last car to pass.
With my route, using 12,000 ton loaded coal trains that have 6 head end locos, 2 rear end helpers, and are 107 cars long, I have added signals so that a train has 3 signals (one protecting front, and one protecting rear, and one in mid train), in other words my train takes up @ 2 blocks (actually 1-3/4 blocks), but the train fits comfortably within the 2 blocks.
Also "Name" your speedboards (ex: "35 Mph EB") so that you don't have to hunt for them, if you decide yo edit them out, or change them.