# 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:** 20\ **Page:** 1
**Author:** ![Spectre\_of\_Pithecanthropus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/spectre_of_pithecanthropus/32/12343_2.png) [@Spectre\_of\_Pithecanthropus](https://boards.straightdope.com/u/Spectre_of_Pithecanthropus)\ **Post date:** [February 9, 2005, 6:38pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/1 "2005-02-09T18:38:28Z")
About 15 years ago my then boss invited everyone up to his place in Palmdale for a barbecue and to watch the first L.A. Raiders game of the season. So I’m driving up I-14 from Sylmar to Palmdale, and I notice a peculiarity about this road, which had three lanes going in either direction (there might be more by now). 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. Lo and behold, a new lane appears gradually on my left. But meanwhile the rightmost lane vanishes, so now I’m driving in the middle lane. A mile or so further on, and now I notice that the lane to my right disappears, and all traffic is forced to merge left. Naturally at the same time, a new lane starts up to the left of the fast lane, and now I’m driving in the righthand lane. Which begins to disappear in due course, and I have to merge left if I want to stay on the road. Am I making any sense here? Basically, the way the road is constructed, you have to constantly merge to the left if you want to stay on it. I’ve never seen any other road like that. Why would they have done it that way? ---
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [February 9, 2005, 6:47pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/2 "2005-02-09T18:47:18Z")
WAG - was this on a steep incline? Was it a route likely to have a lot of heavy-goods traffic? I know of a junctions with a similar setup to what you describe (albeit with only one merging stage), where it’s on a perhaps two-mile constant incline. The provision is for vehicles struggling to get up to an appropriate speed to have more than one opportunity to merge, and for faster traffic to have more than one opportunity to avoide them. ---
**Author:** ![Spectre\_of\_Pithecanthropus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/spectre_of_pithecanthropus/32/12343_2.png) [@Spectre\_of\_Pithecanthropus](https://boards.straightdope.com/u/Spectre_of_Pithecanthropus)\ **Post date:** [February 9, 2005, 6:59pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/3 "2005-02-09T18:59:35Z")
> [@GorillaMan](#): > > WAG - was this on a steep incline? Was it a route likely to have a lot of heavy-goods traffic? I know of a junctions with a similar setup to what you describe (albeit with only one merging stage), where it’s on a perhaps two-mile constant incline. The provision is for vehicles struggling to get up to an appropriate speed to have more than one opportunity to merge, and for faster traffic to have more than one opportunity to avoide them. Could be now that you mention it. This road isn’t particularly heavily used by trucks, compared to some, but then there’s a lot of heavy duty truck traffic on just about every local freeway here. Most of the local rail network, which used to convey freight from central depots to various parts of the city and surrounding area, has been torn up. I can see how the configuration of I-14 would help you get around slower vehicles, but I’m not sure it’s worth the trouble of having to merge left all the time. ---
**Author:** ![butler1850](https://avatars.discourse-cdn.com/v4/letter/b/779978/32.png) [@butler1850](https://boards.straightdope.com/u/butler1850)\ **Post date:** [February 9, 2005, 7:17pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/4 "2005-02-09T19:17:29Z")
The “Big Dig” tunnel on Rt 93 through the center of Boston does exactly this. If you start in the left lane on the South end, heading north, you’ll have to change lanes at least twice in order to come out on the other side. Right lanes turn into exits, and left lanes are created by (some) onramps. I think it’s insane to have a road that you are FORCED to change lanes, even if you are “express” traffic. But that’s a PIT thread. ---
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [February 9, 2005, 8:24pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/5 "2005-02-09T20:24:14Z")
> [@butler1850](#): > > I think it’s insane to have a road that you are FORCED to change lanes, even if you are “express” traffic. 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? ---
**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 9, 2005, 11:05pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/6 "2005-02-09T23:05:33Z")
> [@Spectre of Pithecanthropus](#): > > About 15 years ago my then boss invited everyone up to his place in Palmdale for a barbecue and to watch the first L.A. Raiders game of the season. > > So I’m driving up I-14 from Sylmar to Palmdale, and I notice a peculiarity about this road, which had three lanes going in either direction (there might be more by now). 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. Lo and behold, a new lane appears gradually on my left. But meanwhile the rightmost lane vanishes, so now I’m driving in the middle lane. A mile or so further on, and now I notice that the lane to my right disappears, and all traffic is forced to merge left. Naturally at the same time, a new lane starts up to the left of the fast lane, and now I’m driving in the righthand lane. Which begins to disappear in due course, and I have to merge left if I want to stay on the road. Am I making any sense here? Basically, the way the road is constructed, you have to constantly merge to the left if you want to stay on it. > > I’ve never seen any other road like that. Why would they have done it that way? Fifteen years ago? [When the roads looked like this?](http://members.cox.net/mkpl2/hist/26mp_la.jpg) j/k BTW, it’s not I-14, it’s [State Route 14](http://www.cahighways.org/009-016.html#014) (excellent site regarding highway histories), which may or may not have to meet interstate standards. Hard to say once you roam around this site. I believe they are still trying to improve that freeway and as I recall, going northbound (uphill), they still have those lanes appearing on the left and disappearing on the right. I don’t remember if the same is happening on the southbound (downhill) lanes. If the southbound lanes are like the northbound lanes, then I would believe it is part of the construction scheme to (easily) keep people on the right while they work on the meridian area. If the southbound lanes are not like the northbound, then I would guess that it is to keep slower traffic shifted off to the right on the uphill climb. Either way…kinda goofy. ---
**Author:** ![Hyperelastic](https://avatars.discourse-cdn.com/v4/letter/h/a3d4f5/32.png) [@Hyperelastic](https://boards.straightdope.com/u/Hyperelastic)\ **Post date:** [February 9, 2005, 11:08pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/7 "2005-02-09T23:08:34Z")
> [@Spectre of Pithecanthropus](#): > > So I’m driving up I-14 from Sylmar to Palmdale, and I notice a peculiarity about this road, which had three lanes going in either direction (there might be more by now). [roadgeek] You must mean CA-14. Interstate 14 is still in the planning stages, and is in the Southeast. [/roadgeek] ---
**Author:** ![scotandrsn](https://avatars.discourse-cdn.com/v4/letter/s/a4c791/32.png) [@scotandrsn](https://boards.straightdope.com/u/scotandrsn)\ **Post date:** [February 9, 2005, 11:37pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/8 "2005-02-09T23:37:22Z")
> [@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? The I-405 Freeway out here is notorious for this sort of thing. A lane divides upon the left, but, on the right, a lane is subtracted (perhaps by suddenly becoming an exit-only lane, or simply merging with its neighbor). Thus, if you were in the left lane, you are suddenly in the middle lane of the three-lane road. If this happens again, you are in the right lane. If that right lane suddenly turns up exit-only a few miles later, and you do not intend to exit, then suddenly you, as part of the main traffic stream, must merge with the lane on the left in order to remain on the highway. And they wonder why the roads here are so slow. They blame the population increase, but the large number of cars is simply revealing the inferior lane design. ---
**Author:** ![KGS](https://avatars.discourse-cdn.com/v4/letter/k/90ced4/32.png) [@KGS](https://boards.straightdope.com/u/KGS)\ **Post date:** [February 9, 2005, 11:41pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/9 "2005-02-09T23:41:38Z")
Trying to impose logic on the L.A. freeway system is an exercise in futility. 😛 A similar situation exists on the Ventura Freeway (US-101/CA-134) which at three interchanges loses 3 lanes from the right, while gaining the same number of lanes from the _left._ This means if you’re driving from the West Valley to Pasadena, you need to merge left _six times_ just to avoid getting shunted into downtown. The situation got worse when they added a carpool lane to CA-134 – now, at two places, through traffic gets squeezed into a SINGLE LANE (plus the carpool lane.) As you can imagine, this creates parking-lot conditions even at the best of times, let alone rush hour! ---
**Author:** ![postcards](https://avatars.discourse-cdn.com/v4/letter/p/8e8cbc/32.png) [@postcards](https://boards.straightdope.com/u/postcards)\ **Post date:** [February 10, 2005, 12:37am UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/10 "2005-02-10T00:37:06Z")
> [@Yeticus Rex](#): > > …while they work on the meridian area. And it’s sure a bitch when they work on that meridian. The detours go on for thousands of miles! ---
**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, 7:28am UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/11 "2005-02-10T07:28:14Z")
> [@scotandrsn](#): > > A lane **divides** upon the left, but, on the right, a lane is subtracted…Thus, if you were in the left lane, **you are suddenly in the middle lane of the three-lane road.** Sorry, but I still don’t get it - if the lane _divides_, can’t you just as easily choose to stay in the left-most lane? ---
**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, 1:29pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/12 "2005-02-10T13:29:30Z")
If I recall correctly I-5 coming out of SanDiego, where SR-163 merges does the same thing. So does I-15 right outside of Escondido. I always assumed it was to move the roadbed to the most stable/flat route, without putting in a bunch of curves. ---
**Author:** ![Aro](https://avatars.discourse-cdn.com/v4/letter/a/b3f665/32.png) [@Aro](https://boards.straightdope.com/u/Aro)\ **Post date:** [February 10, 2005, 2:07pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/13 "2005-02-10T14:07:16Z")
I find this quite interesting as I have not seen a road do this where the change of lane you are in would be permanently affected by a turn-off or slip road. Can I confirm that what you are talking about is a main road doing effectively [this?](http://fff.fathom.org/pages/aro/(Road_lanes).jpg) (quick sketch, obviously exaggerated for brevity). Or the mirror of this, considering. ---
**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, 3:23pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/14 "2005-02-10T15:23:39Z")
> [@Aro](#): > > I find this quite interesting as I have not seen a road do this where the change of lane you are in would be permanently affected by a turn-off or slip road. > > Can I confirm that what you are talking about is a main road doing effectively [this?](http://fff.fathom.org/pages/aro/(Road_lanes).jpg) (quick sketch, obviously exaggerated for brevity). > Or the mirror of this, considering. Essentially, yes. What is that, an SVG? ---
**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, 3:31pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/15 "2005-02-10T15:31:44Z")
> [@GorillaMan](#): > > Sorry, but I still don’t get it - if the lane _divides_, can’t you just as easily choose to stay in the left-most lane? 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. 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. As you can see from the posts here, there are so many of these spots in Southern California, traffic is at a crawl most of the day on the busiest roads. That’s as clear as I can make it, and Aro’s diagram illustrates it wonderfully. If the problems in this setup still are not clear to anyone, congratulations! You may have a future with CalTrans! 😃 ---
**Author:** ![Aro](https://avatars.discourse-cdn.com/v4/letter/a/b3f665/32.png) [@Aro](https://boards.straightdope.com/u/Aro)\ **Post date:** [February 10, 2005, 3:34pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/16 "2005-02-10T15:34:26Z")
> [@scotandrsn](#): > > _Essentially, yes. What is that, an SVG?_ The image? It was essentially a cropped screen dump from an very quick dwg on an AutoCAD package I’m currently using. There is only one stretch ofroad near me that does _anything_ like this, but nothing to that extent at all. Must be very tough for visitors to get to grips with. ( Sorry, I realise I’m adding nothing towards answering the OP ) ---
**Author:** ![scout1222](https://avatars.discourse-cdn.com/v4/letter/s/34f0e0/32.png) [@scout1222](https://boards.straightdope.com/u/scout1222)\ **Post date:** [February 10, 2005, 3:54pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/17 "2005-02-10T15:54:01Z")
> [@picunurse](#): > > If I recall correctly I-5 coming out of SanDiego, where SR-163 merges does the same thing. So does I-15 right outside of Escondido. I always assumed it was to move the roadbed to the most stable/flat route, without putting in a bunch of curves. 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. ---
**Author:** ![tremorviolet](https://avatars.discourse-cdn.com/v4/letter/t/e47774/32.png) [@tremorviolet](https://boards.straightdope.com/u/tremorviolet)\ **Post date:** [February 10, 2005, 4:10pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/18 "2005-02-10T16:10:40Z")
OK, I’m a bridge engineer. Don’t have an awful lot to do with road layouts; usually the roadway guys just tell me where the bridge goes. But I do see some of what the roadway designers do. My WAG is that the engineers needed to shift the alignment and didn’t have enough room to lay a proper horizontal curve (I’ve been through the Big Dig and I suspect that’s what’s happening in the tunnels). Roadway layouts have strict criteria as far as degree of curvature for given design speeds, etc. ---
**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, 4:23pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/19 "2005-02-10T16:23:17Z")
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. ---
**Author:** ![Duke](https://avatars.discourse-cdn.com/v4/letter/d/e8c25b/32.png) [@Duke](https://boards.straightdope.com/u/Duke)\ **Post date:** [February 10, 2005, 4:27pm UTC](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238/20 "2005-02-10T16:27:15Z")
> [@Hyperelastic](#): > > [roadgeek] > You must mean CA-14. Interstate 14 is still in the planning stages, and is in the Southeast. > [/roadgeek] Really? Where’s it planned for? [/out-of-the-loop roadgeek] [Next page](https://boards.straightdope.com/t/road-builders-traffic-engineers-why-would-a-highway-be-built-like-this/289238.md?page=2)