I use Spikes on my layout spaced out at 5 inch intervals. In HO, 5 inches travelled in 5 secs - 5 mph, 10 inches in 5 seconds is 10 mph etc. My spikes are painted with every fourth one a different colour to represent the 20mph marks.
You can change the spike spacing to 5.75 inch spacing in OO or 2.75 inch for N scale. The spikes represent survey pegs that are usually left to rot for many years but quite obvious. The faster you go, the more pegs you cover in 5 seconds in multiples of 5 mph which is what most speed limits have. I have a full story about the idea on http://xdford.digitalzones.com/modelrr10.htm but as my server does not like hot linking, you will have to copy the site manually onto another aspect of the browser.
Hope this helps
Regards from Oz
Trevor Gibbs Melbourne Australia