From a great distance, like traffic lights are often viewed from, it is hard to discern shapes--especially square vs. circle.
Glare and flaring tend to reduce the light to a blur anyway (except for those have 20/20 vision and perfectly clear eyeballs viewing them through a clean windshield on a clear day). The color becomes the predominant signal--and therefore it is quantity of light that becomes important.
The above examples all cut down on the amount of light leaving the signal-- especially the "bicycle" and "pedestrian" shapes (which are appropriate for the slower-moving clientele they serve, but would blur into "dim green"/"dim red" for car drivers, especially in the rain.
As @Matt mentioned in a comment to the question, the 'green' traffic lights often have a strong blue component so that RG-colorblind people have a better chance of distinguishing them, the major argument against relying solely on color.
Position provides an additional reinforcement, and a top-down arrangement is clearer than a left-right (or is it right-left?) arrangement, which was the other notably confusing thing about shaped Canadian traffic lights.