What is the purpose of "turn on green arrow only" signs?

[groman]

@Irishman

You can turn left on red from a 2 way street? Seems risky - you are crossing a traffic lane - your oncoming traffic.

Your oncoming traffic is hopefully stopped at that same red light, isn’t it?

[Baracus]

@Irishman

Now I need to go look that up. Consider a left turn lane, needing to U-turn. Left only light triggers, roll up and as I start around the bend, the cross traffic right lane guy is right turning. Even after I’ve started the U and am clearly aiming for that lane. See, it’s a protected Left, so I am in the lane with a green light, where as the right turner is in a lane with a red light. Clearly I have right of way.

Unless the right turner has a green arrow for his right turn, which is usually the case where I see the “U-turn must yield” signs.

[Irishman]

KneadToKnow said:

I drove through the intersection that was on my mind when I posted that just the other day,

Fair enough, but your comment about bottom of the right of way chain was presented as an absolute, not a specific intersection case.

And really, I still don’t see why U turn is bottom of right of way. He’s the one flowing with traffic, with the light, even if it is an unprotected left. The right turner is at a “stop until clear” signal.

groman said:

Your oncoming traffic is hopefully stopped at that same red light, isn’t it?

You are assuming that crossflow traffic is syncronous. JerseyFrank just described the case where flow alternates by direction, so there are four points in the traffic cycle, not 2. My direction goes. Then opposite oncoming goes. Then traffic from my right goes. Then traffic from my left. If the standard default is that you can left on red from a 2 way street, then that case would require additional signage. Of course, in a state with that law, they may take that into consideration and avoid light combinations that create the risk. Traffic engineers may be crazy, but they are probably not that incompetent.

[Mdcastle]

Dragging this up because I noticed this in Chicago yesterday I have an answer for the question. The sign is the city of Chicago being cheap-ass. The intent is to have a protected only turns, but they’re too cheap to put up a proper display of green arrow, yellow arrow, red arrow so they just use the cheaper setup intended for protected / permissive and turn it into protected only by banning the permsisive part with a sign. This practice is absolutely not allowd by the MUTCD, but Chicago just does it anyway.