Horizontal stoplights are a common sight in Texas, Florida, Nebraska, and New Mexico. If you've noticed this, too, you might be one of those who have questioned why. YouTuber Road Guy Rob aims to answer that question by asking the engineers in the said states. Apparently, there are four main design considerations that can lead to the installation of sideways stoplights.
As explained in the video above, one of the primary reasons for using horizontal stoplights is a lack of clearance. In California, a freeway interchange was built on top of an intersection, leaving little room for vertical stoplights. Engineers could have just raised the bridge to solve this issue, but that would have been very expensive. Instead, the stoplights were mounted horizontally to keep the signal head as low as possible.
Secondly, mounting a horizontal signal head requires a shorter vertical pole than a vertical signal head. While saving on metal doesn't save much money compared to bridge construction, the savings do add up with thousands of stoplights installed within a state.
For the third reason, stoplights obviously need to be visible to drivers. Mounting them horizontally ensures they are within the driver's cone of vision – 20 degrees to the left and 20 degrees to the right – making them easy to see without the need for drivers to turn their heads.
Finally, stoplights should be mounted at a height that drivers can easily see, but not too high that they become difficult to spot. Horizontal stoplights can be placed closer to the ground, making them more visible to drivers. Of note, the "Goldilocks Zone" is at least 15 feet off the ground to clear the trucks, but no higher than 25 ft.
The Manual on Uniform Traffic Control Devices (MUTCD) specifies that stoplights can be mounted either vertically or horizontally, as long as they are visible to drivers and not too high or too low. There are no rules about color, orientation, or design, as long as the signal head is between 40 and 180 ft away from the driver's line of sight.