I wonder how much of it might be future proofing (i.e., overbuild in case it has to be opened to vehicular traffic in the future).
I had that thought too, it's weird how similar the bridge span options are despite the vastly different practical requirement between all the options presented. Either the designers are building to assume a retrofit for vehicle access one-day or they have never walked through a successful, attractive and affordable pedestrian/cycling underpass before. Underpasses have real practical benefits for active modes transportation - it's significantly easier and more practical to descend gently 2.5 - 3m rather than do a silly switch back 5m into the air for a pedestrian overpass that takes 4x as long to travel the same distance.
As others have mentioned, I assume that the whole crux of the project from a deliverability and cost perspective is the structure to support the train - shorter spans, shallower depth of underpass is much, much cheaper than a wider spans and deeper underpass. The park and pathway infrastructure is great and would be a cool amenity on either end of the underpass, but costs nothing in comparison.
This is where I keep going back to weird engineering biases that don't seem to be impacted by the public debate on these types of projects. There's a shadowy design influence occurring that favours over-building and future-proofing. For example, a quick google suggests Netherlands has standards of 2.5m depth and active modes underpasses for specific reasons, one being cost. Here's a snip from a design website I think summarizes the things to think about when designing for an active transportation user, which aligns with shallow underpass depth requirements:
- allow for lower gradient and/or shorter approach ramp for cyclist.
- Let the cyclists see all the way through to the other side; to do this reduce the depth of the tunnel.
- Make the approach to the tunnel straight from both sides. Make sure there are no dark corners obscured from view.
- Let the cyclists utilize the speed they gain on the way down to get out of the tunnel quickly.
- Walls leaning outwards helps to create a roomy feeling.
- Consider making “windows” in the roof of the tunnel to allow daylight to enter sections of the tunnel.
- Smooth curves in the construction elements instead of the traditional straight angles can also increase the perception of social safety.
11 Street SW plan proposed 3.0m - because our cyclists are taller than the Dutch? What could be the practical rationale for the extra depth and why is our standard 3.0m?
I will take a guess that an official answer will be about how the extra height has something to do with "comfort" or "perception" of safety in tunnels. But the list of design considerations above do the same thing, they just cost less. Plus if we are so concerned about comfort and safety I encourage transportation departments to turn that thoughtful design to literally all other parts of a car-dominated transportation system we have built, including many dangerous or inefficient designs literally blocks from this underpass.
0.5m difference between 11 Street SW and the Dutch designs sounds small but it's multiplicative all over in the design and use of the underpass. More work to construct, more slope for the future users forever. That kind of thing is baked-in for all these types of projects, active modes or car underpasses alike, that results in overbuilding, higher costs, and in the long run ultimately fewer underpasses because they all end up costing so much.
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