# Road builders, traffic engineers, why would a highway be built like this? **URL:** \ **Category:** Factual Questions\ **Created:** [February 9, 2005, 6:38pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238 "2005-02-09T18:38:28Z")\ **Posts on this page:** 18\ **Page:** 2
**Author:** ![scotandrsn](https://avatars.discourse-cdn.com/v4/letter/s/a4c791/32.png) [@scotandrsn](https://boards.straightdope.com/u/scotandrsn)\ **Post date:** [February 10, 2005, 4:44pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/21 "2005-02-10T16:44:48Z")
> [@Mangetout](#): > > I’m from the UK, where we drive on the left, and as such, I couldn’t quite map the concepts of the OP (you’d think it would be simple, but I couldn’t visualise it), however, it sounds vaguely similar to something I’ve seen on sections of the M25 (London Orbital Motorway) - a new ‘fast’ lane appears and the ‘slowest’ lane peters out - essentially moving all of the traffic towards the ‘slow’ lanes. AFAIK, it is done simply because people hog the ‘faster’ lanes (which are supposed to be for overtaking); in fact it is not at all unusual here to see drivers occupying the middle lane of three when there is little or no traffic in the slower lane and there is a backlog of traffic attempting to overtake them - sliding all traffic toward the slow side is an attempt to make these drivers return to the proper lane. I don’t believe we have that law in California, although I know some states do. ---
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [February 10, 2005, 6:30pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/22 "2005-02-10T18:30:47Z")
> [@scotandrsn](#): > > Perhaps “divide” was a poor choice of words, because it might imply what amounts to a fork in the road, either option of which is as simple to take as the other. What really happens is that the painted lines marking your lane remain more or less unchanged, but the roadbed will widen to the left, where a new lane will be marked. That’s exactly what I was thinking of. What’s so difficult about keeping to the left at such an occassion? > [@](#): > > Now, sane road design would suggest that on a high speed freeway, you would not create a situation requiring constant lane changes to stay on the road. The increased potential for merge situations increases the risk for slowdowns at best, accidents at worst. Assuming drivers are taught properly, there’s nothing dangerous whatsoever in having a road design that requires lane changes. _All_ multi-lane roads require them. And I certainly don’t see how it causes traffic to slow (again assuming drivers know how to do it properly, and how to read the road far enough in advance to anticipate manouveres.) ] > [@Mangetout](#): > > I’m from the UK, where we drive on the left, and as such, I couldn’t quite map the concepts of the OP (you’d think it would be simple, but I couldn’t visualise it), however, it sounds vaguely similar to something I’ve seen on sections of the M25 (London Orbital Motorway) - a new ‘fast’ lane appears and the ‘slowest’ lane peters out - essentially moving all of the traffic towards the ‘slow’ lanes. Where on the M25 does this happen? (Incidentally, my early reply was referring to the crawler lane after the M11 junction, clockwise.) The new section of the M60 round Manchester has another layout designed to deal with people’s failure to use all three lanes - the middle and fast lane are continuous, and at _every_ junction the slow lane becomes the slip road, and the opposite after the junction. ---
**Author:** ![Zelski](https://avatars.discourse-cdn.com/v4/letter/z/c67d28/32.png) [@Zelski](https://boards.straightdope.com/u/Zelski)\ **Post date:** [February 10, 2005, 6:31pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/23 "2005-02-10T18:31:46Z")
I now live in the Seattle area and congestion here is far worse than I experienced driving in Pittsburgh, Chicago, Baltimore, and Cincinnati to name a few cities where I have some driving experience. I have often thought that this type of road design might help this area. Why is that? One of the many driving problems that people have here is that most people ALWAYS move to the fast lane regardless of their speed intent or passing intent. Furthermore they always move left just after they get on. Beyond that they don’t seem to move much. If the driver’s habits here stayed constant and this type of road design was implemented, then after a couple of miles, these very slow drivers would no longer be clogging the fast lane. Another reason to implement this type of design is to avoid forcing together traffic with far different intents. Here in Seattle, people who are going to get off of a highway begin to slow down (to off-ramp speeds) as much as a mile before their off-ramp becomes apparent. This puts those folks in direct conflict with people just getting on to the highway at the previous on-ramp. The people getting on are presumably speeding up (but not in Seattle, but that is a different rant) while the off-goers are slowing down. The type of road design in the OP puts all exits on one side and all entrances on the other, so these two traffic flows do not collide. The most infamous and dangerous interchange in the Pittsburgh area is the eastbound Squirrel Hill interchange where traffic gets off about 1/8 of a mile after traffic gets on. This is exacerbated by the fact that the getting-on traffic must move over an additional lane (or else they get off in 1/8 of a mile) and that a tunnel (which slows people down) is another 1/8 of a mile down the road. Mixing the on and off traffic there has been very dangerous. I’m not saying this design is great (or bad) just that it might address some of the problems created by driver habits. No one should be cruising the left lane and this design moves people out of it. It also might stop the mixing of slowing down traffic and speeding up traffic. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [February 10, 2005, 6:51pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/24 "2005-02-10T18:51:27Z")
> [@GorillaMan](#): > > Where on the M25 does this happen? (Incidentally, my early reply was referring to the crawler lane after the M11 junction, clockwise.) My memory is suggesting the Clockwise, just past the M11 junction, but actually, it might not be the M25 at all; I might be thinking of the M42, or somewhere else altogether - in any case, it’s a stretch of UK motorway that somewhere, in the space of a couple of miles, has five or more instances where a new rightmost lane appears and the leftmost lane peters out, moving the traffic one lane toward the left. ---
**Author:** ![Fear\_the\_Turtle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_the_turtle/32/2926_2.png) [@Fear\_the\_Turtle](https://boards.straightdope.com/u/Fear_the_Turtle)\ **Post date:** [February 10, 2005, 6:57pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/25 "2005-02-10T18:57:53Z")
I’ve only been out here 7 years, so I’m not sure what is was like back then. But I can assure you now the left most lanes (Lane 1 is HOV and Lane 2 would be “the fast lane”) continue undivided from the I-5 split to the AV. Lane 4, however, seems to come and go. I’m not sure if there is a method to that madness, or it is just where the geography was wide enough to permit 4 lanes. I think near Santa Clarita there are 5 lanes in places, but only the rightmost “go away”. Just south of Palmdale it turns to 2 lanes and stays that way (pretty much til you get to Nevada (although it turns into the 395)). Southbound is similar. ---
**Author:** ![scotandrsn](https://avatars.discourse-cdn.com/v4/letter/s/a4c791/32.png) [@scotandrsn](https://boards.straightdope.com/u/scotandrsn)\ **Post date:** [February 10, 2005, 7:24pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/26 "2005-02-10T19:24:03Z")
> [@GorillaMan](#): > > Assuming drivers are taught properly, \ Having grown up in Boston, I can not even begin to think of a more ill-informed assumption by which to plan a metropolitan areas high-speed roads. What are you proposing, that each new driver be given a guided tour of the entire highway system so they can find out and be tested where the disappearing lanes are before obtaining a license? Even if such a scheme were feasible, how would that help in a city such as LA which has voluminous tourist traffic? People get on the highway. If they are going a long way and want to drive fast, they move toward the left, and pretty much stay in their lane, barring avoidance maneuvers, until needing to move right once again before their exit. I have seen computer models showing that the only thing that really clogs up the roads are people slowing down unnecessarily, (such as suddenly finding out that their high-speed lane is comingto an abrupt halt and they must merge over). I say build the roads for the way people use them. ---
**Author:** ![smiling\_bandit](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@smiling\_bandit](https://boards.straightdope.com/u/smiling_bandit)\ **Post date:** [February 10, 2005, 7:50pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/27 "2005-02-10T19:50:36Z")
> [@](#): > > That’s exactly what I was thinking of. What’s so difficult about keeping to the left at such an occassion? Obviously, it’s a problem if you’re not familiar with the roads. Secondly, it’s a real problem for those people keeping right. ---
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [February 10, 2005, 8:08pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/28 "2005-02-10T20:08:45Z")
> [@scotandrsn](#): > > What are you proposing, that each new driver be given a guided tour of the entire highway system so they can find out and be tested where the disappearing lanes are before obtaining a license? What’s wrong with simply having proper signage giving appropriate warning of such situations? ---
**Author:** ![Zelski](https://avatars.discourse-cdn.com/v4/letter/z/c67d28/32.png) [@Zelski](https://boards.straightdope.com/u/Zelski)\ **Post date:** [February 10, 2005, 8:22pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/29 "2005-02-10T20:22:58Z")
> [@scotandrsn](#): > > … > People get on the highway. If they are going a long way and want to drive fast, they move toward the left, and pretty much stay in their lane, barring avoidance maneuvers, until needing to move right once again before their exit. I have seen computer models showing that the only thing that really clogs up the roads are people slowing down unnecessarily, (such as suddenly finding out that their high-speed lane is comingto an abrupt halt and they must merge over)… There are a couple of problems in this paragraph: Yes, if people are going a long way and want to drive fast, they move toward the left, and pretty much stay in their lane, barring avoidance maneuvers, until needing to move right once again before their exit. But, in addition, people who are going a short way and want to go fast AND people who are going a short way and do not want to go fast AND people who want to go a long way and do not want to go fast ALSO move toward the left, and pretty much stay in their lane, barring avoidance maneuvers, until needing to move right once again before their exit. Those people shouldn’t be there and they show no signs of changing their habits. This road design either moves these slow and/or short-way people to the right or makes them pay enough attention to the road that they might notice they are slow and not fast. Next, it might be true that the only thing that really clogs up the roads are people slowing down unnecessarily, (such as suddenly finding out that their high-speed lane is comingto an abrupt halt and they must merge over). While I may detect some sarcasm in the last parenthetical, the thing that clogs up the roads the most are people slowing down unnecessarily, such as, people who are in the left lane and are being undertaken (passed on the right.) if a driver is the leader in the left lane and is being undertaken, then s/he should move right. This road design moves them right. ---
**Author:** ![picunurse](https://avatars.discourse-cdn.com/v4/letter/p/7bcc69/32.png) [@picunurse](https://boards.straightdope.com/u/picunurse)\ **Post date:** [February 10, 2005, 11:20pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/30 "2005-02-10T23:20:46Z")
> [@scout1222](#): > > Substitute I-805 for SR-163 and yes, you’re right. You need to merge left over about 4 or 5 lanes by the time the two routes become one. Really fun during rush hour. Of course, that’s what I meant to say. I left in 1990. When there I lived in Vista and worked at UCSD Medical Center. I-5 to 805 to 163. Thanks… ---
**Author:** ![js\_africanus](https://avatars.discourse-cdn.com/v4/letter/j/87869e/32.png) [@js\_africanus](https://boards.straightdope.com/u/js_africanus)\ **Post date:** [February 11, 2005, 6:37pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/31 "2005-02-11T18:37:13Z")
I emailed the OP to a guy who used to work for CalTrans in the San Francisco area. He said that it didn’t make much sense to him and he’d have to see it to believe it. He said that maybe there was something goofy with the right-of-ways; but he had never heard of such a thing. ---
**Author:** ![Yeticus\_Rex](https://avatars.discourse-cdn.com/v4/letter/y/a698b9/32.png) [@Yeticus\_Rex](https://boards.straightdope.com/u/Yeticus_Rex)\ **Post date:** [February 11, 2005, 7:44pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/32 "2005-02-11T19:44:44Z")
> [@js\_africanus](#): > > I emailed the OP to a guy who used to work for CalTrans in the San Francisco area. He said that it didn’t make much sense to him and he’d have to see it to believe it. He said that maybe there was something goofy with the right-of-ways; but he had never heard of such a thing. Heh, also ask him when they are gonna finish paving the only dirt road section of all State Roads…[CA-173](http://www.westcoastroads.com/california/ca-173.html). He may be scratching his head on that one too. I used to take my son to hockey practice in Lake Arrowhead using this road…7 miles of it was a single lane dirtroad (with a few small turnouts) going up a side of a mountain with cliffs hundreds of feet high (in some places, 1000+ feet!). It was better to drive it at night so you can see headlights coming towards you and find a turnout. Snowstorms weren’t as bad on this side of the mountains than on the southern side (San Bernardino side) because of the orthographical nature of the storms dumped less on this (desert) side. Ah, the memories… ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [February 11, 2005, 7:54pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/33 "2005-02-11T19:54:22Z")
> [@GorillaMan](#): > > I don’t see how this is the case, in the description from the OP - if the fast lane acquires a new ‘faster’ neighbour, then surely the fast-lane traffic is dividing, not merging? I don’t know the road that Spectre was driving on, but here in Dallas there is a case I know of (Northbound Hwy 75 near downtown) where the “add a left lane, take away a right lane” situation happened several times over the course of a couple of miles. This was a few years ago, and I think it’s been fixed now. To answer your question, the lanes added to the left were not just dividing the left lane that was already there, but were instead merged traffic who got their own lane on the left. Then at the next exit, the right lane was a forced exit, followed by another lane merging from the left, etc. To stay in the left lane, you had to continually shift to the left. ---
**Author:** ![galt](https://avatars.discourse-cdn.com/v4/letter/g/35a633/32.png) [@galt](https://boards.straightdope.com/u/galt)\ **Post date:** [February 12, 2005, 1:14am UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/34 "2005-02-12T01:14:40Z")
> [@Spectre of Pithecanthropus](#): > > I change to I-14 and, knowing that I’m settling in for a long drive, I maneuver all the way over to the left, to the fast lane. I suspect that you’re exactly the type of person whose behavior this was designed to counteract. The left lane is for passing, and if you just automatically get over there because you aren’t getting off the highway for a while, you’re blocking it. This type of road design coaxes you over to the right lanes where you oughta be. ---
**Author:** ![brossa](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/brossa/32/3040_2.png) [@brossa](https://boards.straightdope.com/u/brossa)\ **Post date:** [February 12, 2005, 1:56am UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/35 "2005-02-12T01:56:25Z")
I’m afraid that I don’t see how this road design weeds the idiot slow drivers out of the leftmost (passing) lane. If they are idiot slow drivers _because_ they gravitate inappropriately to the left lane, what keeps them from gravitating to the new left lane, veering left each time a new left lane opens up, thereby maintaining their stranglehold on the passing lane? I recall a section of I-5 in Portland, OR, where multiple lanes on the right were being repaved. Traffic as a whole had to be shifted 2 lanes left for several months, using up the leftmost breakdown lane and some of the median. The lane lines were not repainted, so you merged left twice to ‘stay’ in the right lane. Once the repair was completed, however, traffic flow remained shifted one lane to the left - the median went back to its previous state but the left breakdown lane was now the passing lane and the breakdown lane was now on the right; but again, the lane markings had not been repainted so everyone had to ‘merge left’ for a stretch and then ‘merge right’ at the end of the repaired stretch. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [February 12, 2005, 4:54am UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/36 "2005-02-12T04:54:29Z")
That’s nuts; if he’s ever been IN San Francisco, and left via the Bay Bridge, he’s driven on the approach, where the right lanes exit, and new lanes enter from the left – these are on-ramps, but other roads have lanes appearing abruptly on the left. ---
**Author:** ![js\_africanus](https://avatars.discourse-cdn.com/v4/letter/j/87869e/32.png) [@js\_africanus](https://boards.straightdope.com/u/js_africanus)\ **Post date:** [February 14, 2005, 1:14pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/37 "2005-02-14T13:14:41Z")
> [@Nametag](#): > > That’s nuts… I don’t know anything about that; but I do know that I worked with him for three years in a city engineering dept. about ten minutes from where I am currently sitting. I started there in '99, IIRC, and he hadn’t been in California for, again IIRC, eight years or so. ---
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [February 14, 2005, 1:19pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/38 "2005-02-14T13:19:02Z")
> [@brossa](#): > > I’m afraid that I don’t see how this road design weeds the idiot slow drivers out of the leftmost (passing) lane. If they are idiot slow drivers _because_ they gravitate inappropriately to the left lane, what keeps them from gravitating to the new left lane, veering left each time a new left lane opens up, thereby maintaining their stranglehold on the passing lane? It’s not necessarily that they’re _slow_ drivers, it’s just their aversion to changing lanes that clogs the left lane. These same drivers will be slow to make a move into a new lane that appears, thus allowing it to function properly as an overtaking lane for at least these periods of time. [Previous page](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238.md?page=1)