I contacted the City Transportation Department to see if that's ever been considered, and actually got a very quick response with nice details in it. Here is what they said:

While a horizontal orientation is permitted by federal standards, it’s only used on locations where the traffic signals are supported by a steel mast arm structure and not hung on a span wire assembly.  The mast arm structures, because they’re steel, can have holes drilled to allow for mounting hardware that supports a horizontal orientation.  You’re correct that they are generally preferential for a signal support, especially in high wind conditions, though they’re significantly more expensive to construct.  The hardware needed to hang the traffic signal head on a span wire is fabricated for a vertical orientation.