Why traffic lights sometimes sag

Have you ever driven through an area where the traffic lights spanning each intersection are slung so low on their guide wires that they look like they're dangling from the bottom of a hammock?  And you wish you could say to the municipal authorities, "Bubbas, hitch up your signal lights - they're sloppy!" 

I'd show you what I mean, but every time I Google 'sagging traffic lights', I end up with a bunch of hits about sagging pants.  Butt it's a similar idea with traffic lights. 

Screengrabbed from this site. 

Well, it appears that there's actually a method to that madness, a fact I learned only yesterday when I encountered this bizarre scene on Clinton Drive in Houston:

The guy in the oversized Toyota was partially blocking my line of sight, but lo and behold, two guys in bucket trucks had jacked this particular set of loose lights up into the air for reasons that weren't immediately clear. 

But those reasons soon became clear, and so did the mystery of the sagging lights. 

Holy crap - look at the size of that thing!!  I have no idea what was inside Pandora's epic box, but it was headed into the Port of Houston. 

Jacking job done, the bucket trucks folded themselves back up again.  These particular lights are not slung as low as some that I've seen, but it's clear that, if the guide wires had been any more taut, there wouldn't have been enough slack in the system to raise them to a height sufficient to let this thing pass. 

Indeed. 

Before yesterday, it never occurred to me that those lines may have been left loose for a reason.  More than two decades after I started working in Houston's vast and fabulous industrial sector, I'm still learning new things on a daily basis.