I imagine the case for street lighting is even stronger, especially when you start to factor labour costs for bulb replacement as well. And traffic lights would be stronger again, because they're cycling instead of being constantly on, which shortens the filament life significantly.
Now if by "years" you meant "two years", then fine, I can believe it- but two years is still a fantastic payoff period.
Don't forget HPS burns out pretty regularly too. Tactic my muni is using is simply to replace burnt-out HPS with LED instead of repairing it. Lo, after a few years we're probably half LED.
Are you somehow surprised that an infrastructure project would take years to pay off?