# Traffic Managment: Why Do They Do This? **URL:** \ **Category:** Factual Questions\ **Created:** [November 18, 2008, 2:44pm UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279 "2008-11-18T14:44:23Z")\ **Posts on this page:** 16\ **Page:** 2
**Author:** ![Lightray](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lightray/32/391_2.png) [@Lightray](https://boards.straightdope.com/u/Lightray)\ **Post date:** [November 18, 2008, 9:56pm UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/21 "2008-11-18T21:56:36Z")
Hmm… well, no surprise that MODoT did something half-assed. [Here](http://www.mapquest.com/maps?city=Fenton&state=MO&address=State+Hwy+30+%26+Summit+Rd&zipcode=63026&country=US&latitude=38.504139&longitude=-90.456976&geocode=INTERSECTION) is a mapquest map of the intersection. Right-hand turns off of Hwy-30 are _not_ dumped onto a separate lane. That would’ve _really_ helped with the mess at Summit Rd. – (Old) Gravois Rd. intersects Summit right where traffic lines up to turn onto 30. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [November 18, 2008, 11:07pm UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/22 "2008-11-18T23:07:55Z")
> [@NinetyWt](#): > > 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. Sometimes maybe. But there’s one for the gas station at the corner of a main road and a side road near where I work. The entrance is on the side road, the side road goes into a mixed light commercial zone with no homes, and doesn’t get much traffic. Much of the business for the gas station is cars coming from the main road, and going back that way. I think it’s something some traffic researcher came up with, maybe for the reasons you mention, and now locally they just stick it in everywhere because it’s “the newest idea in improved traffic safety”, never mind whether it’s actually appropriate at any given location. ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [November 18, 2008, 11:21pm UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/23 "2008-11-18T23:21:16Z")
> [@ZenBeam](#): > > 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. Standard name is a “right-in-right-out”. They usually put them in because the local zoning prohibits left-ins and/or left-outs. Typically because the entrance way is too close to another intersection, but could also be because they want to keep usage below a certain threshold to avoid putting in a full intersection. It’s not by the property owner’s choice (they want to increase access, not restrict it), but because they probably couldn’t have gotten the project approved without using the right-in-right-out. ---
**Author:** ![NinetyWt](https://avatars.discourse-cdn.com/v4/letter/n/cc9497/32.png) [@NinetyWt](https://boards.straightdope.com/u/NinetyWt)\ **Post date:** [November 19, 2008, 3:41pm UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/24 "2008-11-19T15:41:28Z")
> [@ZenBeam](#): > > I think it’s something some traffic researcher came up with, maybe for the reasons you mention, and now locally they just stick it in everywhere because it’s “the newest idea in improved traffic safety”, never mind whether it’s actually appropriate at any given location. I highly doubt that some evil traffic researcher is putting these things in everywhere. Political decisions made by local officials are probably the culprit. As Pleonast says: > [@Pleonast](#): > > they probably couldn’t have gotten the project approved without using the right-in-right-out. I personally have witnessed my fair share of idiotic projects which do not have the support of the engineering department being implemented because of the Board of Aldermen. Don’t blame the engineer, blame your elected official. ---
**Author:** ![Cervaise](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cervaise/32/16693_2.png) [@Cervaise](https://boards.straightdope.com/u/Cervaise)\ **Post date:** [November 19, 2008, 3:57pm UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/25 "2008-11-19T15:57:01Z")
> [@Lightray](#): > > (I guess I should just be happy they didn’t put in a roundabout, as they did over by my parents’.) A well-designed roundabout is a beautiful thing, and is far and away the most efficient mover of traffic through a complicated intersection. When I was in Italy a year ago, I was unnerved the first time we went through [this](http://maps.google.com/maps?f=q&hl=it&sll=37.0625,-95.677068&sspn=51.222969,79.101563&ie=UTF8&ll=43.896029,10.537707&spn=0.002872,0.004828&t=k&z=18), but by the third time it made perfect sense and I was amazed at how smoothly it functioned to accept incoming traffic and redirect everybody to their various destinations. Yeah, it takes a little getting used to, but it’s an innovation we in the U.S. would be well advised to adopt. (Hardly a new idea, of course, and there are a lot of them in the Northeast, but everywhere else they’re a novelty.) ---
**Author:** ![elmwood](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/elmwood/32/3060_2.png) [@elmwood](https://boards.straightdope.com/u/elmwood)\ **Post date:** [November 20, 2008, 7:49pm UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/26 "2008-11-20T19:49:25Z")
I’m a planner, and I still find myself going “WTF?” at various traffic management practices. Another “why do they do this?” question: In Ohio, when a limited access highway widens from two to three or more lanes, the new lanes are added not on the left, but from entrance ramps on the right; instead of merging into the rightmost traffic lane, they continue as a new traffic lane. Whenever a limited access highway narrows from four lanes to three or two, it’s not by merging the leftmost traffic lane into the remaining leftmost lane, but by turning a traffic lane into an “exit only” lane for the next upocming exit. This forces traffic in the lane to merge left into the remaining traffic lane, causing confusion by drivers unfamiliar with the road, and inevitable congestion. ---
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [November 20, 2008, 8:31pm UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/27 "2008-11-20T20:31:14Z")
> [@elmwood](#): > > Another “why do they do this?” question: In Ohio, when a limited access highway widens from two to three or more lanes, the new lanes are added not on the left, but from entrance ramps on the right; instead of merging into the rightmost traffic lane, they continue as a new traffic lane. …thus providing an opportunity for slower, or slower-accelerating, vehicles to continue without having to merge. It also ensures that the additional lane is fully utilised from its start. > [@](#): > > Whenever a limited access highway narrows from four lanes to three or two, it’s not by merging the leftmost traffic lane into the remaining leftmost lane, but by turning a traffic lane into an “exit only” lane for the next upocming exit. This forces traffic in the lane to merge left into the remaining traffic lane, causing confusion by drivers unfamiliar with the road, and inevitable congestion. If the road is reducing from four lanes to three, then one lane will have to merge into another, however you do it. The arrangement you describe allows this to happen while also providing, effectively, a much longer off-ramp than would be normal, which can surely only ever be a helpful thing, particularly in heavy traffic. What I will accept is that the latter case in particular does depend on clear information being given to drivers at an appropriate distance in advance. These road layouts are perfectly normal on British roads, with the information signs beginning a mile before the junction (compared to 1/2 mile for normal exits), and changes in the lane markings to indicate a separating lane at the 1/2 mile point. ---
**Author:** ![UltraVires](https://avatars.discourse-cdn.com/v4/letter/u/ecccb3/32.png) [@UltraVires](https://boards.straightdope.com/u/UltraVires)\ **Post date:** [November 20, 2008, 8:38pm UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/28 "2008-11-20T20:38:01Z")
> [@NinetyWt](#): > > If you like funky intersections, check [this](http://www.abmb.com/cfi-testdrive.html) out (Continuous Flow intersection). I don’t understand the reasoning here. They tout the “beauty” of the system in that it eliminates a left-hand signal from the process. Great, but it adds a whole other intersection. Not to mention the crazy, old bastard who doesn’t pay attention and goes up to the main intersection and turns left across nine lanes of traffic. I don’t see the benefit. ---
**Author:** ![mack](https://avatars.discourse-cdn.com/v4/letter/m/b19c9b/32.png) [@mack](https://boards.straightdope.com/u/mack)\ **Post date:** [November 20, 2008, 9:02pm UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/29 "2008-11-20T21:02:19Z")
> [@Lightray](#): > > Hmm… well, no surprise that MODoT did something half-assed. > > [Here](http://www.mapquest.com/maps?city=Fenton&state=MO&address=State+Hwy+30+%26+Summit+Rd&zipcode=63026&country=US&latitude=38.504139&longitude=-90.456976&geocode=INTERSECTION) is a mapquest map of the intersection. Right-hand turns off of Hwy-30 are _not_ dumped onto a separate lane. That would’ve _really_ helped with the mess at Summit Rd. – (Old) Gravois Rd. intersects Summit right where traffic lines up to turn onto 30. Yeah, that looks like only a half-CFI intersection. I’m not sure about the concept. It looks like you have to go through 3 lights to fully complete your left instead of only 1 the traditional way. ---
**Author:** ![NinetyWt](https://avatars.discourse-cdn.com/v4/letter/n/cc9497/32.png) [@NinetyWt](https://boards.straightdope.com/u/NinetyWt)\ **Post date:** [November 20, 2008, 10:28pm UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/30 "2008-11-20T22:28:15Z")
> [@jtgain](#): > > I don’t see the benefit. > [@mack](#): > > I’m not sure about the concept. I’m pushing the limit on my understanding of traffic engineering here (I’m a hydrologist), but the linked site says this under “Advantages”: > [@](#): > > Simulations using VISSIM traffic modeling software show that CFI outperforms conventional alternatives dramatically. In case after case, CFI produced extraordinary improvements in levels of service under existing traffic loads and reductions in average intersection delay of 90% or more. > > Achieves significantly more capacity than conventional at-grade intersection designs. > Offers substantial savings over grade-separated alternatives while providing equal or better performance. > Can be deployed in one-, two-, three-, and four-legged versions. > Requires little more right-of-way than a conventional at-grade intersection. > Provides clear lines of sight, avoiding the visual barriers created by overpasses. > Helps reduce pollution by reducing congestion. > Requires shorter construction time and less utility relocation. There are some “testimonials” about the intersection in Baton Rouge [here](http://www.abmb.com/cfi-testimonials.html). I have yet to travel through one, so I don’t know how well they work. Evidently it’s being touted as an economical alternative to overpasses. ---
**Author:** ![crypto](https://avatars.discourse-cdn.com/v4/letter/c/b3f665/32.png) [@crypto](https://boards.straightdope.com/u/crypto)\ **Post date:** [November 20, 2008, 11:29pm UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/31 "2008-11-20T23:29:30Z")
To quote your quote: > [@](#): > > Simulations using VISSIM traffic modeling software show that CFI outperforms conventional alternatives dramatically. In case after case, CFI produced extraordinary improvements in levels of service under existing traffic loads and reductions in average intersection delay of 90% or more. > > Achieves significantly more capacity than conventional at-grade intersection designs. > Offers substantial savings over grade-separated alternatives while providing equal or better performance. > Can be deployed in one-, two-, three-, and four-legged versions. > Requires little more right-of-way than a conventional at-grade intersection. > Provides clear lines of sight, avoiding the visual barriers created by overpasses. > Helps reduce pollution by reducing congestion. > Requires shorter construction time and less utility relocation. What a bunch of crap-ola! I’m sure using the VISSIM software DOES show this wonder of engineering is THE BEST, however it’s a software simulation, not real drivers, (like my mother), who are scared of merging, passing, and a billion other things. Simulators randomly hit brakes? Do they randomly inject a run red light? In a perfect world, this thing might work, but to me it looks like the “unholy mess” that it was described as. Too much going on to confuse just about everyone. Or one. That’s all you need. I’ll bet that VISSIM doesn’t show traffic slowing down before entering a long tunnel either, but ask anyone who drives through one on a dayly commute. Oh, and for that right in, right out unholy triangle, I turned left into one to get into a gas station. Right in front of a state trooper. It was an honest mistake, and the officer let me off with a “that thing causes more accidents than if it was just an open entrance. Just be more careful next time.” THANK YOU, KIND SIR! The other thing he mentioned was if I were to actually enter it legally, then I’d have to drive over 3/4 of a mile for my first chance to legally turn around. (this was in rural Virginia and I had out of state plates.) Just to enter right. WTF? ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [November 21, 2008, 12:21am UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/32 "2008-11-21T00:21:51Z")
**bouv** said: > [@](#): > > 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. Street near where I live, two lane at one part, then widens to three later. Wal-mart moved in at the 2 lane area. Great, the city decides to repave the road. But they don’t widen that portion of the road, even though they strip down to the roadbed. That seemed silly to me, but the repaving project was not driven by the Wal-mart move in, so they didn’t have the funds/approval for the widening. Of course 2 years later they are widening the entire road into a 4 lane divided street with median. And putting in lights. \*\*NinetyWt \*\* > [@](#): > > I personally have witnessed my fair share of idiotic projects which do not have the support of the engineering department being implemented because of the Board of Aldermen. Don’t blame the engineer, blame your elected official. Here’s one. One stretch of road that is one of two paths around a lake. Housing is booming on the south end, so the road is being expanded to handle projected traffic. A two-lane road is widened into a 6 lane road with a doublelane-wide median that is flat with traffic - essentially a double turn lane. There isn’t much in the way of businesses or turns along the stretch of road - yet. The project included widening a bridge. However, there was one oversight. The previous road had a paved shoulder on each side of the street, including the bridge. The current road has no shoulder, including the bridge. There happens to be a surprising amount of foot and bike traffic using this road, as it is 1 of 2 paths around the lake. Yet there are no provisions for foot traffic. Admittedly the lanes are wide, such that a bike can ride in the rightmost lane and still have cars pass it - but I think you have to be suicidal to ride like that. The speed limit is 45 but cars regularly do 60, because the road is 3 lanes each direction and there are no streets, parkinglots, etc along it. Of course traffic isn’t that heavy except during rush hours it can be a bit busy, but still. Especially bad is the bridge, 6 lanes, no pedestrian provisions. So what I frequently witness is pedestrians or bicycles using the double wide middle turn lane. Which is admittedly safer to my mind than using the righthand lane, but still a lot less safe than the previous arrangement with right hand shoulders. And at some point you have to cross three lanes of traffic to get in and out of the middle lane, before the heavily used left turns at either end of the stretch. I’m not sure who to point this out to. I don’t know who “owns” the road. City? County? State? I did call the city and report it, but nothing has happened. **Cervaise** said: > [@](#): > > A well-designed roundabout is a beautiful thing, and is far and away the most efficient mover of traffic through a complicated intersection. Perhaps, but there is something about the perpetual merge that is intimidating. There are always cars trying to pull into your lane from the right while you are trying to get to your right exit. I find this a downside to the interstate feeder roads common around here, the struggle to get up to speed while the car in front of you is trying to slow down, or vice versa. Perhaps it is more efficient when the traffic speeds are consistently lower (35 vs 55), but still feels tricky. **elmwood** said: **Location: Austin, Texas** > [@](#): > > Another “why do they do this?” question: In Ohio,… Hmmm, somebody is lost. 😉 > [@](#): > > Whenever a limited access highway narrows from four lanes to three or two, it’s not by merging the leftmost traffic lane into the remaining leftmost lane, but by turning a traffic lane into an “exit only” lane for the next upocming exit. This forces traffic in the lane to merge left into the remaining traffic lane, causing confusion by drivers unfamiliar with the road, and inevitable congestion. That has merging traffic in the “slower” lanes, not the faster lanes. And people should be more attuned to merging traffic from the right for entrance ramps anyway. But yes, they do need adequate signage. ---
**Author:** ![elmwood](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/elmwood/32/3060_2.png) [@elmwood](https://boards.straightdope.com/u/elmwood)\ **Post date:** [November 21, 2008, 12:41am UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/33 "2008-11-21T00:41:13Z")
> [@Irishman](#): > > **elmwood** said: > **Location: Austin, Texas** > > Hmmm, somebody is lost. 😉 Off-topic: I moved to Austin literally hours ago. I left Cleveland on Monday, and just changed the location field. ---
**Author:** ![Improvisor](https://avatars.discourse-cdn.com/v4/letter/i/50afbb/32.png) [@Improvisor](https://boards.straightdope.com/u/Improvisor)\ **Post date:** [November 21, 2008, 1:16am UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/34 "2008-11-21T01:16:01Z")
Hijack alert > [@Cervaise](#): > > A well-designed roundabout is a beautiful thing Cervaise - if I remember correctly you are a Seattle-area guy… They will be putting in a triple-roundabout in the Woodinville tourist district for your driving pleasure. It’s supposed to be completed next year. [Project Website](http://www.ci.woodinville.wa.us/CityHall/ProjectTouristDistrictRoundabout.asp) [PDF of design](http://www.ci.woodinville.wa.us/Images/City%20Hall/TRIP%20Roundabout%20Project%20Aerial%20Overview.pdf) ---
**Author:** ![NinetyWt](https://avatars.discourse-cdn.com/v4/letter/n/cc9497/32.png) [@NinetyWt](https://boards.straightdope.com/u/NinetyWt)\ **Post date:** [November 21, 2008, 2:44am UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/35 "2008-11-21T02:44:50Z")
> [@Stink Fish Pot](#): > > What a bunch of crap-ola! I’m sure using the VISSIM software DOES show this wonder of engineering is THE BEST, however it’s a software simulation, not real drivers, (like my mother), who are scared of merging, passing, and a billion other things. Simulators randomly hit brakes? Do they randomly inject a run red light? In a perfect world, this thing might work, but to me it looks like the “unholy mess” that it was described as. 1. No where on that site did I notice claims of a CFI being “THE BEST”. As already noted, it is being promoted as an economical alternative to an overpass. 2. Drivers who are scared of “merging, passing and a billion other things” shouldn’t be driving, IMO. 3. I’ve no idea what VISSIM is capable of modeling. I do know that traffic modeling software exists which has the capability to play “what if” and run different scenarios (accident, change in signal timing, blocked lane, etc.). 4. If you are describing the intersection called an “utter mess” by **Lightray** in his above posts, that is not a CFI intersection as shown in my link. > [@Irishman](#): > > I’m not sure who to point this out to. I don’t know who “owns” the road. City? County? State? I did call the city and report it, but nothing has happened. Are you under a ward system? Your alderman should be willing to talk to you about your concerns. If that person doesn’t understand the situation, they might be able to put you in touch with the city or county engineering department. ---
**Author:** ![Green\_Bean](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/green_bean/32/1760_2.png) [@Green\_Bean](https://boards.straightdope.com/u/Green_Bean)\ **Post date:** [November 21, 2008, 4:00am UTC](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279/36 "2008-11-21T04:00:08Z")
> [@Cervaise](#): > > A well-designed roundabout is a beautiful thing… And a poorly designed one is hideous. We had two horrible ones right here. (Clark circle at exit 135 and Westfield circle between North and South Aves.) They’ve improved both of them within the last couple of years, but I was surprised that they didn’t find a way to take them out entirely. Nevertheless, I agree that traffic circles are good solutions in some cases. As far as those CFI intersections: I was marveling at how much extra space they took up–how wasteful, blah blah–until I remembered the jughandles. Oops. [Previous page](https://boards.straightdope.com/t/traffic-managment-why-do-they-do-this/473279.md?page=1)