A Boulevard and a Lid for the Arch Grounds: Here’s How

To some, capping I-70 with a "lid" at the point the Gateway Mall intersects the Arch grounds, and the replacement of I-70 with a boulevard appear to be mutually exclusive. Capping the highway and possibly closing Memorial Drive at the Gateway Mall has been the conventional wisdom in improving Arch grounds connections for more than a decade. Re-envisioning the Arch grounds and Gateway Mall has never really stopped. Since the idea was first put to paper more than three quarters of a century ago, countless individuals and dozens of civic leaders have envisioned how to complete or improve both.

I continue to believe that an at-grade boulevard in place of I-70 from the Poplar Street Bridge to Cass Avenue is the best, most transformative, least expensive, most straightforward design for the Memorial Drive/I-70 corridor. The idea of walking unimpeded from Luther Ely Smith Square to the Arch grounds is inherently attractive. But I think it raises at least as many questions as it answers.

If Memorial Drive is removed between Market and Chestnut (or from Walnut to Pine), traffic would be diverted onto 4th Street and Broadway. Both streets have unused capacity. The streets themselves could handle the traffic. Yet, putting more traffic on these streets further isolates Smith Square and the Old Courthouse. And beyond pedestrians needing to cross Broadway and/or 4th Street, if visitors do what the current Arch design competition wants them to do (stay and explore downtown), they will be crossing more streets. That's not a bad thing. That's a city.

Are we to spend $10M's to cover I-70 and possibly close Memorial Drive for this result? Closing Memorial Drive creates added confusion and detours for drivers, adding more traffic as they navigate one-way streets and dead ends in the heart of a central city – where they do expect to find them. In addition, a lid and the removal of Memorial Drive makes the visitor's experience less predictable. Yes, the entrance from the Gateway Mall (once one makes it to Smith Square) is easier, but then a visitor doesn't know what to expect at the north or south end, making it less likely they will explore further – again, a central demand made by the Arch competition. Alternatively, if a visitor crosses a well-designed boulevard and can see that to the north and south that the boulevard remains the same, with connections at each block, that there isn't an elevated Interstate, or unpredictable access, they will explore. They will stay. They will visit Washington Avenue, the Gateway Mall and the Old Cathedral.

{Spruce Street at I-70 and the Arch grounds – why a lid isn't enough}

That said, this post is meant to explore how a lid and a boulevard together in place of I-70 could work. The important challenge in combining the two elements is retaining all possible pedestrian connections. Specifically this means Walnut and Pine Streets. Sacrificing those connections for a better Market and Chestnut Street connection would represent a victory here and a loss there. Replacing I-70 with a boulevard can work without a lid, in fact two teams leave the current boulevard in place and cover one block of I-70. However, a lid and the removal of parts of Memorial Drive is a awkward and incomplete solution without a boulevard.

Here, I look at removing I-70 and building a boulevard that runs at-grade, dipping underneath the lid for a single block. The intersections of Walnut and Memorial, as well as Pine and Memorial would be at-grade, providing for conventional pedestrian crossings. Market and Chestnut Streets would allow pedestrian access as part of the lid. In this way, quality access is preserved across this three-block span and the boulevard transforms the remaining 1.4 miles. This would be a much preferred alternative to a three-block lid that includes a boulevard because a longer lid and depressed boulevard would be a greater disruption to the street grid.

{lid and boulevard showing Market and Chestnut as one-way streets with underground parking access}

With the myriad of Interstate ramps from the PSB and depressed lanes of northbound I-55/I-44, the existing infrastructure is anything but simple. The regulations and requirements of Interstate gradients, exit ramp radii, clearances, and more are not easy to work with. This is why the PWP-Foster+Partners-Civitas team identified de-designating this corridor a Federal highway a primary task towards building a boulevard. 

The engineering challenges of building a city street in this location aren't easy, but they're remarkably better than an Interstate. The right-of way could be less than 1/2 of the existing width. The street gradient can be somewhat greater, allowing an at-grade boulevard at Walnut to dip below a lid and arrive at-grade at Pine Street. This arrangement would be similar in some ways to Forest Park Parkway at Kingshighway or Forest Park Avenue at Grand Avenue.

By the numbers, the boulevard grade would need to be approximately 6% to accommodate the scenario envisioned above, allowing for 14-foot clearance, the minimum requirement for urban Interstates. This is the same gradient as the maximum allowed for Interstate highways in non-mountainous areas. For reference, cities such as Madison, WI recommend that city streets do not exceed 8%.

In terms of the design team submissions for the Arch grounds competition, SOM and MVVA simply leave Memorial Drive where it is now. PWP move Memorial to only the west side of I-70 and create a two-lane, two-way street primarily for drop-off access for the museum. Weiss-Manfredi remove one block of Memorial and route traffic to 4th Street and back and Behnisch offers a three-block lid and removing Memorial Drive altogether. The lid and boulevard concept would work with each of these designs.

There are several ways in which to access parking under Smith Square. Millennium Park in Chicago has parking access in the median of Michigan Avenue, but I prefer the simple ingress and egress ramps used for the Boston Common underground garage. Such a solution is easily implemented with a boulevard in place of I-70. Of course access to parking from northbound lanes would be possible as well.

Any lid or other infrastructure proposal should be measured by how well it accommodates the future boulevard. Clearly a lid and boulevard can work well together, but if underground parking entrances and exits are built on Market or Chestnut or 4th Street, the opportunity for a better solution will be gone. If infrastructure changes eliminate future pedestrian connections, we will have failed to take full advantage of the once-in-a-lifetime opportunity to reshape our city, the Arch grounds and riverfront.

{access to Boston Common underground garage shown on east side of Charles Street}

{lid and boulevard showing Market and Chestnut as pedestrian areas}