http://letsgola.wordpress.com/2014/09/25/sepulveda-pass-transit-part-3-mode-and-alignment-through-the-pass/
September 25, 2014
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:
- What modes should the project serve? This will determine who can use the project, be it cars, buses, or trains.
- 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 introduct
ory 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.
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.
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.
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%.
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.
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.
Kevin Lee of 
