Sepulveda Pass Transit, Part 3: Mode and Alignment Through the Pass

For an overview of transit between the Westside and the Valley, see Part 1. For a close-up look at LAX, see Part 2.

The most critical part of a north-south transit line between the Westside and the Valley is Sepulveda Pass – the section that roughly parallels the 405 between Wilshire and Ventura Boulevards. Services on the Westside and in the Valley will probably end up having several branches using the pass, in order to maximize the usefulness of the pass segment. Due to the distance (about 7 miles) and engineering challenges, we’re probably only going to get one line through Sepulveda Pass in the foreseeable future. It’s critical that we get this segment right, get the most capacity for our money, and set it up to flexible enough to accommodate many services on both sides.

The two planning questions that must be answered are:

  1. What modes should the project serve? This will determine who can use the project, be it cars, buses, or trains.
  2. What should the project alignment be? This will determine what service patterns can be operated on either side of the pass and how they will relate to each other.

Question 1 comes first, because the mode choice will affect the design criteria for the project alignment, such as curvature, grades, and ventilation.

A Multi-Modal Tunnel?

The concepts that have been floated publicly are all variations on a theme. They propose building a toll auto tunnel that would also provide lanes, perhaps dedicated, for transit. The project is often pitched as a candidate for a public-private partnership.

If the alternative includes a tunnel, I don’t think auto lanes should be part of the plan, for reasons explained here. If HOT lanes are going to be part of the project, they should be converted from existing HOV lanes (or, if you insist on new lanes, new at-grade or elevated lanes, but there’s no spare capacity on the 405, the 10, and the 101 for new lanes to connect to anyway). That leaves bus and rail.

The primary trade-off between bus and rail is implementation timeline versus capacity and operating costs. If the corridor is for buses, it can be used immediately by many bus services connecting all parts of the Valley and the Westside, while a rail link from Wilshire/Westwood to Sherman Oaks would be of limited use in isolation. Choosing rail would delay the usefulness of the project until feeder lines were built on both sides. However, as passenger volumes increase, which we would expect for a useful Sepulveda Pass project, rail offers higher capacity and lower operating costs.

Four options come to mind:

  • A guideway exclusively for buses
  • A guideway exclusively for rail
  • A hybrid guideway running both buses and trains (not as crazy as it sounds; Seattle is running a tunnel like this right now)
  • A larger guideway with four lanes, two for rail and two for bus (or hybrid)

The first two options just seem underwhelming for the context. We’re not talking about the Gold Line from Azusa to Claremont or an improvement to an arterial corridor that’s got parallel arterials to be upgraded a mile away on either side. This is it – the one big project between the Westside and the Valley that we need to facilitate more growth between Sylmar and Long Beach. You don’t want it to end up like the MBTA Green Line, right?

Capacity Counts

Some more serious numbers: in the post on capacity, we estimated about 5,000 pax/hr per direction for bus (standing load, 60 second headways) and 15,000-20,000 pax/hr per direction for LRT (standing load, 2 minute headways, 3 or 4 car trains). For comparison, the five lanes of the 405 (we’re ignoring the climbing lane and auxiliary lanes) have a capacity of about 12,000 veh/hr per direction. Obviously, the passenger capacity depends on how many people are in each car; assuming 1.2 pax/veh (not unreasonable for commuting), that’s 14,400 pax/hr per direction.

That gives you an idea of the magnitudes of how many people can be moved by each mode. You can vary the assumptions as you like (double articulated buses, longer trains, higher occupancy in cars). Bus headways below 60 seconds are probably beyond the point where rail offers higher reliability and lower operating costs. The inclusion of bus would be mainly motivated by the desire to put the facility to use immediately, without waiting for long branch rail lines to be built.

That puts a transit option with one lane in each direction in the same league as the existing 405, so maybe that’s enough. On the other hand, the relentless congestion on the 405 suggests there’s a crap ton of latent demand – in other words, a lot more people would be traveling through Sepulveda Pass if it were easier to do. We want this project to relieve the 405, but also to facilitate economic growth on the Westside and in the Valley. With that in mind, a large diameter tunnel with four tracks may be the way to go.

To see why we might want a tunnel with two lanes or tracks in each direction, consider the effect of branching. Since Sepulveda Pass is a natural bottleneck, we should be serving several parallel north-south transit lines, bringing them together for a trunk through the pass and allowing transfers. In the opening post, we identified up to four corridors on each side to be served. With an operational headway of 2 minutes and one track in each direction, that’s 8 minute headways on the branches. This is short of Metro’s design criteria, which calls for operational headways of 5 minutes on LRT branches. With a large diameter tunnel and two tracks in each direction, operational headways of 4 minutes would be achievable on the branches.


In the introductory post, I defaulted to the assumption of a tunnel the whole way from Westwood to Sherman Oaks. Alon Levy rightly called that assumption out in the comments, prompting a look at some elevated and hybrid options.


An elevated option is self-evidently going to follow the 405. This is both the best horizontal alignment and the best vertical alignment that does not involve a tunnel.


From a technical standpoint, the critical section of the alignment is the approximately 1.5-mile long 5.5% grade on the north side of the pass. Light rail vehicles (LRVs) can handle short 5%-6% grades without issue; in fact, there are 5%-6% grades in many places on the new Expo Line for grade separations. However, I’m not sure if vehicle braking performance would suffer on such a long downgrade, and it might be difficult to support the required headways.

Let’s assume 2 minute operational headway and 90 second design headway (Metro’s current design criteria for a trunk LRT line is 2.5 minutes operational and 100 seconds design). Safe braking distance, for signal design, must include (a) distance traveled during reaction time, (b) braking distance, and (c) a buffer between vehicles. If you’re using fixed signal blocks, the buffer might be the vehicle overhang; for Communications Based Train Control (CBTC), let’s use an assumed imprecision in the system’s knowledge of where the vehicle is located.

Metro’s current design criteria specifies 9.8 seconds of reaction time. This might seem like a lot, but it has to cover equipment reaction time, operator reaction time, and brake build up. This value isn’t atypical in US practice. For braking, Metro specifies a distance of 0.733*S2/(B+0.2G), where S is speed, B is the braking rate (assumed to be 2.0 mphps), and G is the profile grade. Let’s assume 200’ for vehicle location imprecision (more precisely, 100’ for each train, with the worst possible combination of errors.

For a design speed, let’s assume 60 mph. For safe braking, you need to assume the entry speed when braking starts is higher due to a combination of speedometer error and equipment tolerance. To keep things simple, let’s assume 65 mph. That yields a reaction distance of 934’ and a braking distance of 3441’, for a total of 4575’ (including the 200’ CBTC buffer). Using 0.2G underestimates the effect of gravity a little; if you calculate the braking distance based on a 2.0 mphps braking rate adjusted by the laws of motion, you’ll get 5029’.

Okay, so that’s the separation you need from the rear of one train to the front of the train behind it. If you want the theoretical headway, you need the distance from the front of the train to the front of the train behind it. In other words, you have to add the length of the train. In this case, that’s four 90’ LRVs for 360’. If you have fixed signal blocks, you also need to add the length of one clear block of track, as shown below, but since we’re assuming CBTC, we’ll ignore that distance.


That gives a total distance, based on Metro criteria, of 5389’. At 60 mph, that’s 61 seconds of travel time, essentially a 1 minute theoretical headway. Even if you assumed fixed signal blocks and added a clear signal block distance, it would seem that a 2 minute operational headway is within the realm of possibility.

Note that this is still a simplification; the headway impact of having a station, presumably at Ventura Blvd, at the bottom of the grade would have to be determined by simulation. This analysis also ignores other potential physical constraints, for example the ability of the LRV to continually put out maximum braking force for that long or the impact of wet rails, that wouldn’t be an issue on shorter grades. Premature rail wear, such as rail corrugation, might occur. These issues are well beyond my experience. (Hint, hint, technically inclined commenters.)

From a route planning perspective, the elevated alignment is not ideal at either end. At the south end, you end up at the 405 and Wilshire, west of the proposed Wilshire/Westwood station on the Westside Subway. It wouldn’t be too hard to deviate west to the Veterans Hospital; however, this is bound to be a low demand station. Wilshire/Westwood is a much better location for the transfer, because it will eliminate the need for many people on the north-south transit lines to transfer in the first place. It wouldn’t be too hard to get over to Veteran Av by crossing the cemetery (they’re the abutters least likely to complain). That makes the transfer reasonable, but still puts the stop at the very margin of UCLA and Westwood. From there, the line would probably head back towards Sepulveda, but more on that another time.


At the north end, the first stop would naturally fall at Sepulveda/Ventura. North of there, the line could hop over to Sepulveda Blvd at the 101 or at Burbank, and follow Sepulveda north through the Valley. Sepulveda is good corridor, and deserves a high quality transit service, but most of the interest in the Valley seems to prioritize Van Nuys over Sepulveda. Getting from Sepulveda to Van Nuys would require a one mile jog to the east, and the resulting zigzag would be bad route planning. However, Sepulveda/Ventura is a decent node in its own right.



A hybrid alignment would follow the same route as the elevated alignment from Wilshire to the 405 just north of the Sepulveda Blvd ramps. This would require about 3.5 miles of tunneling, just a little more than half of what the full tunnel would require.


This alternative would save some money over the full tunnel alignment, because elevated construction is usually cheaper than tunneling. It would also allow the northern approach to be constructed at a much gentler grade, around 1.0%, than the 5.5% grade required by the elevated option, and greatly reduce the length of the 3.0% grade on the southern approach.

From a route planning perspective, this alternative is also somewhere in between the elevated option and the full tunnel option. The southern end would suffer the same drawbacks as the elevated option, but the northern end would be in a better location, as described under the full tunnel option.


The tunnel alignment would follow the approximate route of the tunnel that has been proposed publicly, from Wilshire/Westwood to Ventura/Van Nuys. This route would be in tunnel the whole way. It might be possible to build some of the route at-grade through UCLA’s campus, but it’s probably not worth the effort to bring the line to the surface for such a short distance.

This alternative would cost the most, but it would have the best track geometry, with a ruling grade of 1.0%.


From a route planning perspective, it’s also the best option at both ends of the alignment. At the south end, it puts the Wilshire/Westwood stop in the right place for both transfers to the Purple Line and for local destinations at UCLA and Westwood.


At the north end, it lines up perfectly with Van Nuys, the highest priority north-south corridor in the Valley, and yields reasonable geometry for additional branches to the west towards Sepulveda, Reseda, and Balboa.


Boring Questions

Assuming a tunnel is going to be part of the selected alternative, the cross section of the tunnel is the next question. With the exception of the Blue/Expo Line tunnel on Flower Street, all of the transit tunnels in LA were constructed with the same cross section, consisting of two single-track tunnel bores, connected every so often by emergency cross passages. The stations are center platforms located between the two bores.

For Sepulveda Pass, you’d have a few options:

  • Four single-track bores, built in pairs either simultaneously or sequentially. In this option, you would probably build two tracks at the outset, leaving the next two tracks as a future project.
  • Two two-track bores, again likely leaving the second set of tracks as a future project.
  • One four-track bore.

Alon Levy pitched large diameter tunnel boring machines (TBMs) as money-savers because the station platforms can be located inside the bore; I’m not sure how much they’d save for an LA-type station, relative to the costs of the additional excavation.

However, I think a large diameter TBM might make sense for the Sepulveda Pass project for different reasons. For one thing, when you do two single-track tunnels, you have to make a decision about how many TBMs to buy. Do you buy two TBMs, at considerable up-front capital expense, and allow both bores to proceed simultaneously? Or do you buy one TBM, and bore each tunnel sequentially, paying the price of a longer construction schedule? Using a larger diameter tunnel means buying fewer TBMs and a shorter construction process.

Personally, I like the idea of one four-track bore with two tracks on each level. One level could be used for rail right from the outset, with the other level used for express bus services between the Valley and the Westside. In the future, the bus level could be converted to rail if needed for capacity. The advantages in time and cost are many: construction of launching pits is only needed once, the full capacity is available after completing one bore, working near an active transit line is avoided, and labor costs are reduced by minimizing complexity and shortening the duration of construction. This approach also avoids the tendency of future capacity improvements to remain forever in the future.

Some recent examples of large diameter tunnels include the M30 freeway in Madrid (inner diameter 44.13’), Line 9 in Barcelona (inner diameter 35.8’), and the Alaskan Way tunnel in Seattle (diameter 56’). The TBM in Seattle is, of course, currently broken down, but don’t let their crummy execution sour you on the concept of a TBM that large. Barcelona Line 9 was apparently built to be just large enough for a four-track section, to allow crossovers between stations, but that seems like a really tight section for four tracks. On the other hand, 56’ would probably overdo it and result in high costs for the launching pits and excavation.

A 45’ diameter tunnel would allow four tracks, along with space for breathing room to fit in mechanical and electrical equipment. In particular, with a long tunnel like Sepulveda Pass, it might make more sense to set up the ventilation like a freeway tunnel, with continuous clean air and polluted air levels below and above the travel ways, respectively. In contrast with most transit tunnels, which depend on the piston effect, this design would hopefully allow the ventilation system to meet the requirements of NFPA 130 without restricting the system to one train per direction in the tunnel between stations. Such a restriction would cripple a long tunnel’s capacity to the point that building it would be almost pointless. (The NFPA 130 requirement is actually one train per tunnel vent zone; relying on the piston effect means that each length of tunnel between consecutive stations is operated as one vent zone.)


The space to the sides of the tracks would accommodate electrical and mechanical systems, emergency egress, and ventilation as needed.

For an overview of large diameter tunnel costs, see this post on long freeway tunnels.


There are several feasible alignments and mode alternatives through Sepulveda Pass. While an elevated facility following the 405 is theoretically cheaper, it may be less so in this case because it would have to be constructed over and around an active freeway. The hybrid and full tunnel options offer better routes, and might be worth the trouble, especially if a high capacity tunnel can be built in one bore (and we can reign in US tunneling costs a little). An option that has provisions for both bus and rail will allow higher utilization of the tunnel from the beginning, without needing to wait for all the branch rail lines to be finished.


22 thoughts on “Sepulveda Pass Transit, Part 3: Mode and Alignment Through the Pass

  1. devin

    Enjoyable posts! I have a couple comments:

    1. Aren’t they still considering Wilshire/Gayley for the Purple Line stop? That’s already a less car-congested block, and it’d be right next to the UCLA bus center for an easy hop up to campus–plus no further from most offices (closer to the federal building!), and nearby to the flyaway. These are minor concerns mostly, but make the 405 routing easier than a Wilshire/Westwood stop.

    2. While a 405 => Van Nuys route is an ugly zig-zag at first, it’s basically identical to the Tunnel => Sepulveda (valley) zig zag that we are expecting to build later, no? Meaning it’s only ugly while it’s the only thing on the map: once we build out the network and the 405 route becomes a bottleneck/transfer point, the Van Nuys zig-zag will look totally natural! Or am I missing something?

    3. Come on, give us a UCLA stop separate from Wilshire/Westwood!! It’s over a half mile up to the future conference center and transit plaza, it’ll make a perfect stop =)

    1. Joseph E

      3. Agreed! UCLA deserves its own stop on campus. The ridership will be high enough to justify it. The station can be built partially open to keep costs lower, if the University is willing to deal with the hassles of open cut construction.

    2. Fakey McFakename

      Re TBM costs, would Metro be buying it or just leasing whatever it needs? If it’s buying, then the cheapest is presumably whatever is compatible with what it’s using for the Purple Line (so, if the Purple Line is accelerated, it can use the same TBM; if not, then at least they can share parts).

      And I don’t think you can divorce the engineering from the financing. If Metro can’t afford to build the ideal thing right away, it needs options that allow it to build something and then make the improvements gradually later. (By the way, has Metro ever done studies of using Mello-Roos/CFD/county general obligation bond financing?)

      1. Fakey McFakename

        Also, it looks like the Westwood/UCLA Purple Line station will have entrances at both Westwood and Gayley, with the platform basically under Gayley. With about 400′ between the streets, that’s easily within a commuter’s platform change distance.

        By the way, aren’t there potential 4(f) and general political issues with a route through a national cemetery? I’m guessing any alignment would have to be TBMed deep under the cemetery to avoid the political and funding mess that would come if the plan involved disturbing remains.

      2. letsgola Post author

        Not sure if anyone has ever tried Mello-Roos for something like this… interesting idea. I’m sure there are political issues with the cemetery. If you have to TBM through there, but you want to try to go elevated through the pass to save money, you’d have to surface somewhere, and I’m not sure where it would be – not a lot of ROW. Maybe north of Sunset?

    3. letsgola Post author

      1. I’m not sure where the stop is planned to be, but the platform would be long enough to extend from Westwood to Gayley. Westwood really is better for serving more demand; if your stop is at like Veteran & Kinross, the whole west side of the station area is filled with people who aren’t ever going to ride.
      2. It’s an 80 degree zigzag versus a 100 degree zigzag… so maybe it’s not thaaaat much worse.
      3. While that would nominally violate my 1 stop per mile rule, universities are such strong generators of transit demand that I’m inclined to agree. It would save north-south riders from having to deal with the Wilshire/Westwood station, and it would improve passenger flow for the transfer by diverting some non-transferring demand to a different station.

  2. AJ

    Given the capacity needs, light rail doesn’t really make sense for the corridor — three car trains would be at crush loads, and five car might be too. Sepulveda Pass needs a heavy rail subway from the Orange Line to LAX that provides speed and plenty of room for growth.

    1. letsgola Post author

      The section through the pass needs a really high capacity, but on either side we probably want to branch into a couple corridors. The corridors on either side probably don’t need heavy rail type capacity. I think it’s better to serve two (or more) LRT branches on each side using the pass as a trunk; with heavy rail we’d probably have to pick only one corridor, leaving demand on the others unmet.

      1. anonymouse

        The problem with branching though is that it hurts reliability, and bad reliability hurts capacity. The highest capacities are on heavy rail lines with simple, non-branching service patterns, because the shorter the headway, the less opportunity for recovery there is if one of the trains coming into the core section is late. And the more branches there are, the less correlated the delays are likely to be. Just look at the current Expo/Blue shared section on Flower, or the aforementioned MBTA Green Line. The latter especially has a lot of problems due to unpredictable travel times on the surface, combined with short headways in the core. They try to solve some of the problems by padding the schedule with a 4 minute wait at Kenmore (where most of the lines converge), but that brings its own problems.

        Branching could still make sense, if there really were two equally-strong North-South corridors on both sides of the pass, but I think the better plan is to rely on LA’s grid to distribute ridership on either side of the pass: have good connections to the completed Wilshire and Expo lines in the south, and to a Ventura Blvd BRT/light rail/subway/whatever and the Orange Line in the north.

  3. Joseph E

    “Light” rail can have almost as high capacity as “heavy” rail, if you build the platforms long enough and make the electrical substations strong enough. The new light rail line in Ottawa to 6 cars, which should be able to carry 1200 people per train or 25,000 people per direction per hour at planned frequencies. The problem in finding enough room for 960 foot long platforms in the surface-running sections. Even 4 can trains can carry 800 people per train. If the rest of the line is build to handle 24 trains per hour, that is 19200 people per hour per direction.

    1. Fakey McFakename

      Someone should design a LRT car that allows people to move between cars. Means you don’t need whole train to fit on platform.

      1. Joseph E

        Moving between cars is great, and light rail can be designed that way, same as metro rail can – if you are willing to give up the ability to change the length of trains during the day or week. But platforms still need to be long enough for every door to open directly onto the platform. Otherwise, dwell times at stations increase by a huge amount of time; this slows down the train and limits the capacity of the line. It’s cheaper to make the platforms longer, as long as any elevated or underground stations are built to allow long enough platforms in the first place.

  4. Phantom Commuter

    You know…Westside NIMBY’s are going to oppose any project that can bring more people and development into their sacred enclave ? 🙂

    1. letsgola Post author

      Sigh… I mean, Expo Line did get built. But we are going to have to fight pretty hard to get development that would allow Expo Line get used to its full potential.

  5. Josh

    A better example of a large diameter tunnel completed in the US is the Port Miami Tunnel. For Sepulveda I would keep the twin bores. Each bored housing a combo of rail and auto. Here is why.

    For example you would have the upper decks in each direction for two lanes of auto. Then you can still have cross passages and help with ventilation. Lower level would be two tracks of rail or bus depending how you initiated the configuration. Clearance on rail would be less than with auto so you should be able to fit it with something less than Seattle.

    Make the receiving pits large enough to support a launch back. It would take a couple years in each direction but you have probably the premiere tunnel project in the world and something that would last 150 years plus.

    A job this size I wouldn’t let Metro or Caltrans touch. Run it through a P3 and see the innovation take care of the finance and engineering.

    1. letsgola Post author

      My main problem with allowing cars is that it’s going to drive the ventilation requirements way up, probably increasing costs more than you could recoup with tolls. (On the operations side, I’m really not sure what we’d connect the auto lanes to on either side… if we cleared out the pass, we’d turn the 101 and the 405 in the Valley into worse messes than they are today.)

      1. Josh

        I agree you have to have somewhere for the vehicles to go on either end or you are just moving the problem.

        That is where the only solution would be HOT managed lanes. That is where the future of transportation funding is headed wheather we like it or not.

        As far as ventilation there are solutions that are a drop in the bucket compared to the TBM drives which could including treatment prior to exiting tunnels.

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  7. Adam

    how does the sepulveda pass tunnel compare to the only known long tunnel in the area, the tunnel from Hollywood Highland to Universal City? What were the costs of that tunnel? What were the ventilation requirements of that tunnel? What were the soil conditions required for that tunnel? What was the terrain above the tunnel like? How long did construction of the tunnel take, what were the pitfalls?

    Also, given the recent hysteria in hollywood over the capital records skyscrapers, there’s going to be hyper earthquake concerns on any proposed route, what are the earthquake fault lines like in the areas proposed for the tunnel?

    I feel like the Red Line tunnel should be used as a baseline to give us a local reference point we can grasp, it might also be good for debunking the absurd 8 billion claim for the tunnel. The sepulveda pass tunnel should probably be about the same per mile cost as the Red Line Tunnel plus inflation.

    1. letsgola Post author

      I need to try to dig into info on the costs, but broadly, the terrain should be similar. If I recall correctly, the overburden for the Red Line tunnel maxes out around 1,000′, which is about what Sepulveda will be. Hollywood/Vine to Universal City is ~3 miles and change, about half the length of the Sepulveda Pass route. The fault line that has caused so much hysteria in Hollywood extends west and would likely have to be crossed somewhere; note that the Red Line crosses the same fault. The “Shamrock Shake” occurred at a location not previously known to be a fault, so there may be additional faults to be discovered. One thing to note is that tunnel boring machine technology has improved since the Red Line was built; modern earth pressure balance TBMs are better at preventing settlement.

  8. orulz

    I like the large diameter TBM idea. The idea that all of these routes can possibly be built as rail within the next century is frankly a fantasy, so BRT should absolutely be a component of the plan.

    Pick the one Valley route with the largest ridership potential, probably Van Nuys, and build that as a dedicated heavy or light rail line with no branching under the pass and down Sepulveda to LAX. The other N/S bus routes in the Valley on Reseda, Balboa, and Sepulveda, should then be channeled down Ventura and into the BRT tunnel. Emerge at Wilshire/Westwood and distribute from there, maybe down Wilshire to Santa Monica, Westwood to Culver City, and maybe Santa Monica towards Beverly Hills. Ridership in the BRT tunnel would probably be higher initially but as travel and development patterns adjust to the presence of a rail line it would catch up.

    Not a big fan of ending the Purple Line at Wilshire/Westwood or turning it north to the Valley. IMO it should continue to Santa Monica.

  9. Ian Mitchell

    “A hybrid guideway running both buses and trains”

    I think this is the crux of it, and hybrid should be the approach- 405 for part, tunnel for other. One big bore, start with bus service, add LR once it can connect to something, and if LA and transit ridership continue to grow eventually it’ll need all the trunk capacity there for rail. But quick-start is very important here.


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