Tag Archives: Westwood

Sepulveda/LAX Transit Part 4: Arrested Arterials, Arrested Development

Los Angeles has a great arterial grid. Almost.

As Jarrett Walker says, grids are great for transit, because they let you efficiently serve a large geographic area with high-frequency service, while avoiding the problems of branching. In that post, Walker calls Los Angeles and Vancouver “two of the most perfect transit cities I’ve ever seen, in their underlying geography.”

Well, almost.

Aside from the obvious topographical barriers like the Santa Monica Mountains, there’s a weakness in LA’s arterial grid: on the Westside, practically nothing west of La Cienega is worthy of being called a north-south arterial. East-west movement is another story, an embarrassment of riches: Pico, Olympic, Santa Monica, Wilshire, and Venice. There’s not one north-south arterial that’s as good as any of those east-west boulevards. This is part of why the 405 is so horrendous and traffic on the Westside in general, the largest bugaboo of Westside NIMBYs, is terrible. Most importantly, it has significant implications for what high quality north-south transit on the Westside will look like.

Let’s take a closer look. The map below shows how many lanes each Westside arterial has in each direction. It also shows peak-hour only lanes (created by parking restrictions) with dashed lines. I realize many readers may not be particularly worried about auto capacity, but the number of traffic lanes is a reasonable proxy for both the technical and political challenges to creating a dedicated right-of-way (ROW) for transit.

Westside-v1

How Did We Get Here? Or, A Boulevard for Our Kingdom

It wasn’t supposed to be this way.

Believe it or not, parts of the Westside were undeveloped as recently as the end of World War 2. Older urban nodes, like Palms, Santa Monica, Westwood, Venice, and Beverly Hills, were scattered among farmers’ fields and country clubs. After the war, with construction of the 405 and UCLA, the areas in between these nodes rapidly developed as single-family neighborhoods. Partly due to preexisting development, and partly due to poor planning foresight, adequate ROWs for north-south arterials were not reserved. (Note that this mistake is rarely repeated nowadays; suburban sprawl is planned with ridiculously wide ROWs saved for arterial roads.)

Despite having been developed with single-family residences (SFRs), at that time much of the Westside was zoned for multiple-family development. As discussed previously, by the 1950s, low-rise apartments known as dingbats were popping up in the more established neighborhoods, such as Palms, and in the area near UCLA.

By the early 1960s, high-rise residential towers were sprouting up along Wilshire Blvd, as detailed in the city’s 1963 Westwood Economic Study. By the city’s generous standard of high-rise (4+ stories), there were 19 buildings in this class completed or under construction. The study noted that “rents in the newer high-density apartments [were] considerably higher”. The average monthly rental for a one-bedroom in a high-rise in 1963 was $2,370/month; compare this to one-bedroom dingbat apartments in Oakland, renting for $820/month at the same time, or in Palms, renting for $610-$820/month in 1970. (All values 2014 dollars.)

However, the study went on to note that there was no reason to worry about these expensive rentals, because “increasing demand for apartments on the part of the lower-middle and low income groups. . . can be satisfied in part by the older portions of the existing housing supply, and in part through new smaller apartment structures which could be built at lower cost. Such structures are being built in quantity throughout the city and metropolitan area. . .” Yep, you know who that is.

The rapid changes on the Westside came with growing pains. For example, the Palms-Mar Vista-Del Rey Socioeconomic Study (1970) found that population increased from 19,399 in 1940 to 78,134 by 1956, after which time further growth required replacement of SFRs, some scarcely more than a decade old, with multi-family developments. In 1950, Palms-Mar Vista had 2,361 apartments and 11,859 SFRs; by 1970, 15,883 apartments and 13,100 SFRs, with the number of SFRs having peaked in 1960 and by then declining as they were replaced by apartments.

Probably the biggest change, though, was Century City bursting onto the scene in 1963, rising from a former Fox studio backlot. Along with continued growth at UCLA, Westwood, and Santa Monica, the Westside was becoming one of the most important business districts in Los Angeles. The Westside’s boom strained the area’s underpowered road and transit network, resulting in worsening traffic congestion. In particular, Century City is poorly located and laid out with respect to north-south traffic movement. (Perhaps its developers quite logically expected redevelopment of adjacent golf courses, allowing Avenue of the Stars to be extended north and south.)

The city did not sit idle in the face of this congestion. While the need for rapid transit along the Wilshire Corridor was already well-recognized by the early 1970s, the plans drawn up for north-south movement reflected that auto-centric era. City traffic engineers proposed widening many north-south roadways, including every one shown on the map above between La Cienega and Lincoln (except Sepulveda, which presumably was already an arterial). These plans were backed by City Councilman Louis Nowell, a notoriously pro-growth politician who, in addition to greater urban density, also supported questionable amenities like the Laurel Canyon and Beverly Hills Freeways, and oil drilling in Pacific Palisades.

Westside SFR owners, having bought into the suburban dream only to find themselves in the midst of a booming metropolis, understandably did not share Nowell’s vision. By November 1972, the LA Times reported that opponents of density and street widening, dubbed “environmentalists”, had succeeded in getting the city to cancel plans for Castle Heights-Beverwil and Motor in the West LA plan area. The West LA plan was approved in December 1973. Further west, the Times reported that by early 1974, Mar Vista residents were protesting street widening in their neighborhood. By December 1974, city planners had relented, only proposing to widen Bundy-Inglewood and Barrington-McLaughlin-Slauson. Unsatisfied, voters elected a slate of “environmentalist” candidates to the city council, which proceeded to eliminate all north-south improvements save Centinela in early 1975.

Nowell complained bitterly, arguing that the West LA Plan would “[turn] this city back into a bike and horse economy, like some cruddy little town” and predicting that the Palms-Mar Vista-Del Rey Plan would turn LA into a “cow town because you can’t get around”. The city traffic engineer warned that “if people think traffic in the area is bad now, they ain’t seen nothing yet”. City Councilor Marvin Braude, one of the “environmentalist” councilors, argued that they had shifted the planning process on the Westside “to such things as car pools and rapid transit”, a claim that the last 40 years of history has proven dubious under the most generous definition of the word. Other opponents of development posited more metaphysical arguments, such as “suppose they widened the streets and no cars came”.

Ultimately, widening was restricted to Robertson, Overland, and Bundy-Centinela, though vestiges of the planned wider roads can still be found from Beverwil to Barrington. This requires the city to try to coerce Century City traffic into traveling over a mile east or west before heading south to access the 10.

Opponents of development also secured significant downzonings with the West LA Plan and Palms-Mar Vista-Del Rey Plan. The latter plan, calling for preservation of SFRs, restricted zoning for apartments to areas where they were already being developed, primarily the area of Palms east of Overland with a few tendrils extending to the west. This was expected to reduce the population at build-out from 168,000 to 130,000. (Unfortunately, the articles don’t say how many units this represented; obviously, regional prices and rents will have an impact on household size, and consequently on population.)

The West LA Plan likewise downzoned areas that had been zoned for multi-family development, and expected to reduce population at build-out from 135,000 to 106,000. Around the same time, the permissible density of Century City was reduced, including a reduction in commercial space as well as slashing the number of residential units allowed from 8,000 to 4,000. The density allowed on Wilshire Blvd was also reduced.

By 1975, a new plan was in place for the Westside, one that envisioned a future of lower density and less traffic, but would only deliver on one of those promises.

Fast Forward to Today

There would be further challenges to urban density in the intervening years. For example, the Braude and Zev Yaroslavsky-backed Prop U (1986), reduced allowable FAR in most of the city’s commercial zones from 3 to 1.5. While Prop U targeted commercial development, C1 & C2 zones allow R3 & R4 uses by default, so Prop U effectively reduces residential development opportunity as well.

Century City commuters stubbornly refused to use Overland and Robertson, leading to further resident outcry over “cut through” traffic. But despite the promises of shifting planning away from the automobile, rapid transit and pedestrian/bicycle plans have also been stymied since that time. Plans for a subway on Wilshire were delayed for years by Henry Waxman’s 1985 ban on federal funds for subway construction on the Westside, and Zev Yaroslavsky’s 1998 ballot measure that precluded use of Measure A and Measure C funds for such purposes. The Expo Line will open soon, but only after overcoming several lawsuits from Cheviot Hills and Rancho Park residents. Westside streets remain auto-oriented, with some lacking sidewalks, and bike lanes being opposed by resident groups like the Westwood South of Santa Monica Homeowners’ Association.

Where Do We Go From Here? Or, Our Kingdom for a Boulevard

It should be clear that, in the modern framework of thinking about cities, there are no clean heroes or villains in this story. On one side, you had support for greater urban density, to be enabled by widening streets all over the place and hacking the Laurel Canyon, Beverly Hills, and Slauson Freeways into existence. On the other side, you had strong “environmentalist” support for eternal SFR neighborhoods without any meaningful support for rapid transit, resulting in innumerable hours of traffic misery for people forced to commute on the 10 and the 405.

For our purpose here – north-south transit on the Westside – we need to understand this history, and how it affects the transit options that are available. Congestion and the uneven street grid make it difficult to lay out effective, efficient transit lines on surface routes, but the low density neighborhoods call the practicality of fully grade-separated transit into question. From a transit perspective, the two biggest gaps in the arterial network are probably the lack of a direct north-south connection from Century City to points south, and the incomplete Barrington-McLaughlin-Slauson arterial. The former makes it difficult to plan any north-south transit service to Century City, including connection to the Expo Line, the only high-quality rapid transit service in the near term. The latter would allow for a nice surface BRT route from the Slauson Corridor to West LA along roads with no ramps to the 10 or the 405, avoiding the congestion caused by such facilities.

This post is not to say that all these street widening projects should be resurrected. US cities seem to be unusually adept at destroying themselves that way, though other cities since the time of Haussmann have shown you can have wide boulevards that are an urban amenity, and arterials that will hopefully have high-quality transit someday, like La Cienega, came into existence that way. The takeaway here is that the north-south roads, as they exist today, are the reality we have to work with, and they are going to make it difficult to get exclusive or semi-exclusive ROW. It’d be convenient if we had an Olympic Blvd, but we don’t. More on the potential north-south transit services next time.

For opponents of development on the Westside, the question is more philosophical, and I’d put it like this: you’ve been tilting at the windmills of traffic and density for 50 years now. But no neighborhood is an island. Century City can’t be unbuilt. UCLA and LAX aren’t going anywhere. Santa Monica, Venice, and Playa Vista are some of the hottest commercial markets in the region. People living in the Valley and South Bay are always going to have reasons to travel to the Westside; service industry workers are always going to need a place to live and a way to get to work. How much longer are you going to keep fighting a battle that can’t be won the way you’re fighting it?

Sources

Westwood Economic Study, City of Los Angeles (1963)

The Low-Rise Speculative Apartment, Wallace Francis Smith (1964)

Palms-Mar Vista-Del Rey Socioeconomic Study, City of Los Angeles (1970)

“Palms-Mar Vista Preliminary Plan Calls for Apartment Zone Rollback”, LA Times (January 23, 1972)

“West LA Plan Proposals Face Bitter Opposition”, LA Times (June 29, 1972)

“Resident, Developers Angered by Proposed Plans for Century City”, LA Times (August 13, 1972)

“Planners Will Tackle WLA Traffic Problems”, LA Times (November 12, 1972)

“Traffic Fears Delay WLA Plan’s Approval”, LA Times (November 19, 1972)

“West LA Plan Foretells Severe Traffic Congestion”, LA Times (March 11, 1973)

“West LA Plan Approved Over Protests Noise Was Ignored”, LA Times (November 15, 1973)

“West LA Plan OKd, Called Move Away From Autos”, LA Times (December 20, 1973)

“Mar Vista Resident Group Hits City Plan”, LA Times (February 24, 1974)

“Palms-Mar Vista Plan Approved”, LA Times (December 26, 1974)

“Environmentalists ‘Sweep’, Community Plan Wins Approval”, LA Times (April 24, 1975)

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.

Alignments

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.

Elevated

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.

Sepulveda-405alignment

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.

headway

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.

Sepulveda-south-elevated

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.

Sepulveda-north-elevated

Hybrid

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.

Sepulveda-hybridalignment-markup

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.

Tunnel

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%.

Sepulveda-tunnelalignment-markup

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.

Sepulveda-south-tunnel

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.

Sepulveda-north-tunnel

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.)

Sepulveda-xsection

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.

Conclusion

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.