Thanks Clifford,
I'm after a commercial quantity so I have put some emails around to some companies and I will see what that comes up with.
Ultimately I think I need brighter LEDs and I am now looking at some kingbright LEDs that are advertised as being suitable for traffic and parking signs.
I am yet to have a really good close look at our local 60km/h to 40km/h changing speed limit signs. I thought that they had some sort of filter, but that may not be the case.
The whole bright LED thing was a little worrying - while I am happy with PWM driver circuitry, I wonder what the long term reliability effects are of driving leds higher than 20ma. Its difficult to say before actually trying it and seeing how long it lasts.
I am also looking at changing the red led colour to a green. From the datasheets that I have seen, Green leds appear to throw out more mcd per dollar.
I am yet to find a really good difinitive example of what exact mcd rated leds (and what brand of led) are used in what signs.