Traffic Managment: Why Do They Do This?

Every once in a while I come across weird traffic patterns. OK, most of the time I see weird traffic patterns. Two left turning lanes, a straight lane, and a right lane when the majority of traffic the majority of the day goes right. Two straight lanes and no right lanes when the majority of traffic goes right.

Most of the time I figure the city was bought off or some other devious plot, as these suspiciously retarded intersections are always by big businesses or shopping centers. I did read an article once suggesting that most traffic patterns were not thought out.

But, somewhat randomly, lying in wait near nothing overly suspicious, there’s intersections with the most unholy of unions: the left/straight lane + right-only lane combo. I’ve never understood this intersection. Even the most common of common sense should tell you that right and straight need to go together, because they have the right-of-way, while lefties are hanging out on their own (maybe we’ll toss them an arrow if they’re nice). What invariably happens at these intersections is the people who are going straight get in the right-only lane. This confuses people in the right-only lane, slows down traffic, increases the number of accidents, and confuses people on the other side of the intersection. One place I used to live had done a repaving of an intersection with this problem. When they repainted the road, things got funny: someone didn’t get the message about the stupid intersection and united straight and right-only. Of course traffic flowed perfectly right up until someone caught the “error” and had it repainted. :frowning:

Is there some school of thought in traffic design that suggests this intersection somehow makes any sense? Rarely do I think traffic designers are actually stupid, they probably do have better information than I about traffic flows throughout the city instead of at one or two intersections, but this one is a massive stumper.

Capacity for each lane (movement, at intersections) is something which is calculated by traffic engineers. It’s likely that they were sized for peak traffic at the peak time of year. Do you notice these lanes being used to their maximum during, say, the Christmas shopping season? If you are seeing these empty lanes near big shopping centers, that may be one reason. The lanes may stay mostly empty the rest of the year.

Hopefully someone else more knowledgeable about intersections will come in and explain to you about the other kind of intersection. I will point out that the design of intersections and signals is dependend upon a myriad of things: traffic count being only one of them. Other intersections in the area and their proximity to the one in question have a significant effect, for example.

If you like funky intersections, check this out (Continuous Flow intersection).

Which, to me, is stupid. When I see an intersection where a significant number of drivers make a right turn from a straight-only lane, someone screwed up, and it ain’t the drivers. Special circumstances like holiday shopping are why we pay police officers overtime to direct traffic. But anyway, that’s a rant… the real question is the unholy combo of left/straight that boggles the mind.

That is awesome. Thanks!

Many intersections might also have been made/designed several years ago, when traffic patterns were different. It is now too expensive/time consuming to remake the intersection. Especially when voters will complain left and right that “they really should do something about this traffic,” and then vote no on all tax increases to the highway funds. :rolleyes:

I’ve been interested in traffic patters for a long time now, mostly because I want to know what the hold up is when I’m driving home. My amateur interest has lead me to believe that it is a FAR more complicated task than appears on the surface. I’ve probably read hundreds of websites, emails and other types of content related to the topic and still don’t really understand it.

Just an anecdote:
Years ago, a big high-end shopping center was built in downtown Scottsdale. The city decided (or was bought off) to change the traffic flow on Scottsdale Rd. (the main N-S road), so that there was a dedicated turn lane into the parking garage. This had the effect of royally screwing up the traffic flow on Scottsdale Rd. The shopping center turn out to be a complete fiasco, and everyone involved lost their shirts. It’s now a school and, whadda’ you know, - the dedicated left turn was removed and traffic is back to normal.

If you join straight-right, people in the right lane might go flying by, but left rarely gets the hole in oncoming traffic they need to turn safely. The people waiting to turn left build up and cause congestion in both lanes. By joining left-straight, people turning left tend to go in pairs as people waiting to turn left each reach top of the queue on opposite sides of the intersection.

If you model it, you find out that the left-straight union produces steadier traffic flow for all motorists.

I second the OP. As a bicyclist, if I am turning left, legally I must use the left/straight lane, but unlike every other lane combination, there is no safe way to do this.

If the lead car signals left, I get on its right, signal my own left, and hope the people behind me going straight choose not to run me over. If the lead car does not signal, I get on its left, signal my own left, and hope that it will not secretly turn left and run me over.* Alternatively, I can wait in line for my turn, massively slowing and inconveniencing everyone behind me, and hope that the people going straight will not run me over in their righteous anger. Finally, I can lurk at right hand side of the lane, wait until all cars have passed, and hope I can make it through before the light turns red, assuming no yellow- or red-light runners will come through and run me over.

Usually, when I come to one of these, I now just go straight across, illegally switch to pedestrian status for a moment and align myself with the perpendicular traffic, and then go straight across again, which is less efficient and less legal but also less dangerous and less of an annoyance to the drivers.

*I got chewed out by a non-signaling policeman once for doing this. He pulled me over on the other side of the intersection and gave me a lecture on safe cycling. I gave him a lecture on the use of the turn signal. We were both nice about it, so no shouting or tickets ensued.

Ugh. There is one of these intersections on my way home. The back-up from people trying to turn left at rush hour frequently extends through the two lights leading up to this one. The particularly galling thing about this intersection is that there really is no need for a left turn onto that street. It is only one block long and dead ends at the next major road, and there are far, far easier ways of getting to anything on that block.

The frustration from this intersection is the reason I give myself for going to happy hour and letting things clear.

After happy hour I only need to deal with the trains…

What peeves me is when they put in a new stop light for a gaddamn parking lot, when there are already stop lights a half block away on either side.

Also, they have an annoying habit of putting up walls on overpasses so that you can’t see how traffic is on the freeway in order to decide whether or not to use an alternate route. And for that matter, closing off alternate routes to traffic chokepoints with one way signs or “no right turn 3:30-6:30 PM” and the like.

I think traffic engineers should be made to drive through the messes they make.

That’s supposed to be based on traffic count. If the count for the vehicles coming in and out of the lot exceeds a certain amount, a signal is required. Of course that may not be justified in the particular case you mention.

Do you mean noise barriers or Jersey rail? I don’t think traffic engineers have anything to do with either one.

To be fair, many times folks are trying to design the best they can under pre-existing adverse conditions. It’s hard to make a silk purse out of a sow’s ear. :wink:

Funky does not come close to describing that mess. They recently put one in near where I live (Gravois Bluffs, for St.Louisians) – it made left-turns into a death gauntlet. People turning right never think to check if there is oncoming traffic in the CFI Leg who are completing their left-turn. Near-misses are constant, accidents are frequent, and the people living nearby now bitch about all the horn-blowing from irate drivers. Utter mess.

Hmm. Sounds like they need a “no right turn from red”. I wondered how that was going down there - what kind of traffic counts go thru that intersection, I wonder? It’s my understanding that one is going to be constructed in Lyman, MS also.

I think the traffic count through that intersection is only going to increase – by a lot; they just opened a JC Penney’s at Gravois Bluff, and are building a theater complex, more restaurants, and several strip malls. So the left-turn dealie will be seeing much more traffic, probably by next year’s holidays.

I suspect that they need a traffic light controlling the right-turn lane… but I also suspect that no one in MO would think to look for a light there, since we’re so used to right-on-red.

(I guess I should just be happy they didn’t put in a roundabout, as they did over by my parents’.)

  1. Not justified. It’s a small garage, but it belongs to a law school, which I strongly suspect has a lot to do with it.

  2. They are jersey walls that have been built doubly high for the express purpose of not allowing people to see if traffic on I-66 is backed up. (this was actually in the local paper). The prevailing thought around here is to force people into bottlenecks rather than allow them to seek alternative routes.

  3. I was just venting about that one… :slight_smile:

To agree with An Arky, there does seem to be a tendency of traffic engineers to over-engineer traffic. The thing I’ve been seeing lately is “directed”* entrance/exit drives, where a triangular median is added in a driveway (for a gas station, for example), and on either side of the median, the drive is only one car wide. The lanes are angled to accept traffic coming from the oncoming direction, and direct traffic in the same direction. Looks like a “Y”, with the road running along the top of the “Y”.

It works great if you’re making a right turn in, and then continuing in the same direction when you leave, but on average, that’s only 1/4 of the customers. I don’t know what they’re thinking.

  • There’s probably some standard name for these.

Traffic engineers have an answer for everything, and it usually involves “believe it or not it would be worse any other way”.

That would irritate me as well.

That sounds like one of these things which is meant to prevent turning movements the opposite way. In some cases it’s for safety (too many vehicles coming down the main street to safely turn that way) and in some cases it’s to prevent traffic on some other street. An example of the second case is one of our suburban communities here, where residents are adamant that nobody “drives thru our neighborhood”. That results in businesses near the entrances of these subdivisions with no way to turn back into the subdivision. I tried to point out one time that it would prevent me as a resident from being able to easily go in and out of the store, but that fell on deaf ears.

According to the link, right-turners have their own lane that only joins the main road past the point where the left-turners are crossing the main road. There should be no way that right-turners would interfere with left-turners at all. Is there no such path in the intersection you describe?

Just noticed that, in the linked design, traffic coming opposite the left-turners also has stop bars and signals to prevent them from colliding with the left-turners.

Lightray, sounds like y’all only got part of the intersection you needed.