Brightness is a function of amount of light and area illuminated.
For more light obtain as many lumens as you can afford.
For less illuminated area use LEDs with low "radiation angle" or "cone angle".
If that is not bright enough you can use a lens or reflector.
Many companies provide lenses for LEDs.
Deal Extreme have a range here
Effect of colour: Note also that brightness is related to eye response to colour - yukky yellow green is about the best colour for optimum eye perceived brightness - but if you want a specific colour this is not much help in optimising brightness.
Related: Mind plasticity add on for the ever young:
Lime Green is close - Lime Green Fire Engine from here - BUT more recent studies suggest that familiarity is more important than visibility for emergency vehicles.