physical - Traffic light, no more shapes?

I remember when I was a kid. Each traffic lights had different shapes. Red = Square, Yellow = Triangle, Green = Circle. Now all the traffic lights that I see are round.

Is there a reason why they don't need to have different shape any more?
Isn't it harder for people that have trouble with colors?

(Source: http://en.wikipedia.org/wiki/Traffic-light_signalling_and_operation#mediaviewer/File:Colourblind_traffic_signal.JPG)

_{(source: dec-ced.gc.ca)}

Answer

1. Its more expensive to manufacture and maintain different shaped lights.


2. Round hoods on each light provide better shielding against sunlight and better protection against rain (quicker dissipation) and wind by being more aerodynamic (less resistance).


3. Parts are more interchangeable. Examples: One hood fits on all the lights. The clear tempered glass cover on the red light also fits on the other lights. Clear glass is less expensive than colored glass.


4. Even color-blind people know where the red, yellow and green lights are located.


5. (not relevant to this question) Each light is now made up of a matrix of small LED's. If one LED fails, the rest keep on working. As we know, LED's are brighter, more energy-efficient and last much longer than regular bulbs.