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Laura rants about traffic lights - Qualified Perceptions
Laura rants about traffic lights
So, these signals have been bothering me for a while. The first one, I finally partially figured out, but I still think it's inefficient. The second one, is just WRONG.

Anyway. This is the intersection of the one-way Dover Street with Mass Ave, in Cambridge. At the intersection, Dover is two lanes - one left turn lane, one right turn lane.

When it's time for Dover Street to go, you get a green right arrow only first, and then a green left arrow only. At neither time does any other lane have a green light. The only reason for this somewhat perverse splitting up of the leftgoers and the rightgoers is that the crosswalk is on the right, and they get a walk light during the left-only light. But there's no reason that the leftgoers couldn't be going while the rightgoers are already going. There's no crosswalk on the left, and none of the cross traffic is going. Also, I think this is more concern for the people in the crosswalk than Mass Ave shows anywhere else - pretty much everywhere else, it's perfectly happy to have the walk light on a straight-plus-turning-included green. And the crosswalk two down has no lights at all, it's just "cross when it doesn't look like anyone is going to kill you." Not that I object to doing something nice for the pedestrians, but it surprises me because it's so unusual. I do object to being inefficient with the leftgoers, though. :)

The other Intersection of Inexplicability is four blocks down, at Cameron and Mass Ave. Let me describe the intersection. Mass Ave is two lanes on either side of the divider. Cameron Ave, to the right, is one of the more major smaller roads - it's a snow emergency route, it's the first through street past the bike path, it has a lot of traffic. To the left is Harvey Street, which is tiny and one way away from Mass Ave. I used to say that I never saw anyone turn left onto Harvey Street, but while I was hanging around taking pictures, I did finally see one person do so.

Anyway. When the light turns green on Mass Ave, the northbound folks briefly get this set of lights before the full green:

I can kind of see the point in giving someone the chance to take the left before the oncoming traffic gets to go, in case there's a lot of traffic. But why give the left to Harvey Street, rather than to the people going south who might want to turn left onto the much more travelled Cameron? But that's not even the real question. The real question is, why on earth is there a left arrow from the right lane? You can have two lanes of left turning people when you have a lot of people wanting to turn left and the leftmost lane is a dedicated left-turn-only lane. (Like going from Mass Ave left onto Somerville Ave, in Porter Square - there's a left turn lane and then a left-and-straight lane and then a straight lane. But it makes no sense at all here. The left lane isn't left turn only. Almost nobody turns left. Harvey street has only one lane, it doesn't have room for two lanes worth of left turners at once. Turning left from the right lane would be the worst idea ever. So.... what the heck?


18 comments or Leave a comment
jadia From: jadia Date: January 29th, 2012 08:23 pm (UTC) (Link)
Traffic signals around here are insane. It's the only place I've seen both red and green lights at the same time and it took a while to figure out! The northbound mass ave light near Harvard Sq. is also horribly confusing in the same way that the Dover St. light is - what I like best is when you have red and yellow at the same time. :p

Incidentally, for the 2nd intersection that you are talking about, the southbound traffic does actually get a dedicated left onto Cameron - I use that fairly often. :) It just happens *after* the northbound traffic gets to go.

I also didn't think that 2 traffic lights would mean one is for each lane - I just always thought it was to increase visibility of the lights, just in case. (In case of what, I don't know...crazy drivers, maybe.) In fact, there is a road near where I work (Hartwell Ave) which has 2 traffic lights on it, but the road is only lane-marked as 1 lane. (Practically speaking though, it's actually 1.5 lanes, and it's used as "2-lanes-with-no-shoulder" during rush hour.) The fact that the road has 2 traffic lights side by side was used as an argument that it's actually a 2-lane road, despite having no lane markers. (I think now it's been marked as a 1-lane road right where there are 2 traffic lights...talk about confusing.)
firstfrost From: firstfrost Date: January 29th, 2012 10:32 pm (UTC) (Link)
I guess I am not completely wedded to the idea that two lights and two lanes means one light per lane, but... either the lights are different, in which case they probably ought to be taken to apply to the lane they correspond with, or the lights are all the same, in which case they ought to be taken to apply to all the lanes.

I can't convince myself of a situation in which there are two lights and two lanes, and both lights are meant as instructions for the left lane, and neither applies to the right lane. :)

(I could accept one lane two lights, or two lanes one light, without quibbling too much. Though one lane two lights where the lights are different would be kind of baffling.)
kirisutogomen From: kirisutogomen Date: January 30th, 2012 12:38 pm (UTC) (Link)
How about, both lights are always the same and apply to both lanes? Generally we AND our traffic instructions together; I'm in the right lanes, so I'm allowed to {turn right, go straight, stop} and the traffic light has green plus a green left arrow, so I'm allowed to {turn right, go straight, turn left} and the intersection of those two sets tells me what I'm actually allowed to do.

Having both lights always the same means that if for whatever reason one light isn't visible -- maybe a bulb burned out, maybe part of a vehicle is obscuring it, maybe a giraffe on a scavenger hunt stole it -- people still get the signal information.
firstfrost From: firstfrost Date: January 30th, 2012 12:52 pm (UTC) (Link)
Okay, I can accept that as a definition that makes sense, but I do not think that this definition is commonly implemented in other signalage in the area.

I do not think that in most intersections, the signals above your lane give you indications to do things that you are not permitted to do in that lane. I can think of lots of examples where the left turn arrow is on the left signal only, and no others where the left turn arrow is displayed all the way across (except when all the lanes can turn left). Or the red-straight-green-right you see sometimes, the green right is only displayed on the rightmost lane.

kirisutogomen From: kirisutogomen Date: January 30th, 2012 01:30 pm (UTC) (Link)
Yeah, I have no idea if there are any other intersections using this ruleset. I drive north through that Mass/Cameron every time I leave your house, but I'd never noticed there being anything unusual about it.
firstfrost From: firstfrost Date: February 1st, 2012 05:44 am (UTC) (Link)
You wouldn't really notice the weirdness unless you were first in line - the left arrows are only there for about a second or two.

I did get an answer from hr_macgirl's person, and he said you and I were both right. :) You, because always have redundant signals for redundancy. Me, because that was a not a good place for the backup copy of the left arrow. It might move!
kirisutogomen From: kirisutogomen Date: January 30th, 2012 12:14 pm (UTC) (Link)
Red and yellow together are supposed to indicate that someone has pushed a pedestrian crossing button somewhere, and I believe that it's illegal to turn right on a red+yellow even when it would ordinarily be allowed to turn right on red.
firstfrost From: firstfrost Date: February 1st, 2012 05:48 am (UTC) (Link)
I have never seen that myself, though I did see it in Wikipedia also. :)

The RMV handbook has a picture of a red and yellow light for "this signal is broken", but I think it may mean red *or* yellow (flashing).
kirisutogomen From: kirisutogomen Date: February 1st, 2012 12:15 pm (UTC) (Link)
Oh, I didn't see that in Wikipedia; I've seen it in real life many times, although not for a few years now. I had no idea it was peculiar to Massachusetts.
dcltdw From: dcltdw Date: January 29th, 2012 11:37 pm (UTC) (Link)
Dover St: there's a cross-street to Mass Ave ahead and to the right from Dover that has signals at the same time. I believe they have the same signals -- so if Dover has the R arrow, the opposite street also has R arrows. This is because if it was just a generic green, the L turning people out of Dover would not necessarily expect R turning people, because that traffic is not precisely opposite them.

Cameron Ave, I'm not familiar with since they changed that intersection.
firstfrost From: firstfrost Date: January 30th, 2012 04:01 am (UTC) (Link)
Oh, that makes sense, thank you! :)
There were never any cars coming from that offset cross street while I was stalking the intersection, so I was ignoring them as a source.
kirisutogomen From: kirisutogomen Date: January 30th, 2012 12:11 pm (UTC) (Link)
Note that this explanation only makes sense if the lights for southbound traffic on Mass Ave aren't actually at the intersection with Dover St, but are at the intersection with Hollis St (the cross street that's opposite and slightly offset north relative to Dover). As that's actually true, this makes perfect sense.
hr_macgirl From: hr_macgirl Date: January 30th, 2012 01:21 am (UTC) (Link)
These lights are in Cambridge, yes?

I'm emailing you with the details of The Guy to contact in Cambridge to ask about this intersection. He's the Traffic Engineer for the city responsible for traffic signals. He can explain every signal and the reasoning behind the cycle.

mathhobbit From: mathhobbit Date: January 30th, 2012 03:00 pm (UTC) (Link)
Why did you leave out the light I have to go through to park at your house, at which traffic going in two different directions has the right-of-way. (I think it's left turners and the crosswalk they're turning across that share right-of-way, but that intersection is so confusing that I've never been sure.)
firstfrost From: firstfrost Date: January 30th, 2012 03:20 pm (UTC) (Link)
Well, turners and crosswalk both having the right of way is pretty common anywhere you don't have a dedicated walk cycle. Davis Square is confusing because it's a six-way intersection, but that particular bit doesn't seem unusual if you think of that lane as having the same mechanics as a standard 90 degree left turn - first you get a green left arrow while the oncoming traffic in the other direction is stopped, and after that you get a plain straight green (and lose the right of way), and you can turn left only if there's space.
brass_rat From: brass_rat Date: January 31st, 2012 06:46 pm (UTC) (Link)
I think others have covered the comments that I would have made about these two intersections. Neither of them actually bother me that much.

The one I hate (and I'll admit I haven't checked that its still broken recently) is by the Galleria in East Cambridge. If you're traveling west on Cambridgeside Place, at 1st street, when you get a green light, you see a green right arrow and a green left arrow (and no green circle. When I asked the city about it, they said it was to help alleviate people going straight which would put them the wrong way on a one-way street. The problem is that east bound traffic also has a green light then, meaning that you have a green left arrow, but the left turn isn't actually protected.

I also think that every light on Oxford street could be replaced with either a 4-way stop, or a yellow-flashy in one direction and a red-flashy in the other and greatly improve things, but I haven't actually talked to the city to find out whether there's something I'm missing (pedrestrian traffic at the park being the main one I can think of).
firstfrost From: firstfrost Date: February 1st, 2012 05:54 am (UTC) (Link)
Ouch. I can't imagine what the right signalage would be for the traffic cycle you describe (left or right only, for the love of God do not go straight, but your left is not protected), so it seems like the actual traffic cycle should be altered so that it fits with what can be signaled.
kirisutogomen From: kirisutogomen Date: February 1st, 2012 12:17 pm (UTC) (Link)
I think the correct answer involves a flashing "Do Not Enter" sign.
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