4. Costs for a Freeway Cap:
Conceptually, it doesn’t seem like a freeway deck should cost much more than a bridge or the substructure for most high rise construction. However, cap projects tend to involve significantly more soft costs, especially regulatory costs, than traditional development projects. Perhaps these costs will come down in the future as cap structures become more common and municipal agencies and DOTs plan for them. Additionally, building in an active right of way (ROW) involves costs not encountered in land-based development. On the other hand, airspace ROWs are unlikely to involve significant site contamination clean-up. However, private development will likely involve collection and filtration of auto-emissions.
In the future, there seems to be significant potential for cost reductions in highway cap construction. As the number of projects increase, so will design breakthroughs and construction experience. In particular, highway cap projects supporting intensive commercial or residential development present intriguing possibilities. Buildings will not simply be constructed on top of platforms. Rather, building frames will be integrated with – and be essentially one with – the cap platform. Thus, rather than viewing such developments as building the land (i.e., the platform) in the highway airspace upon which buildings may be constructed, the project may be viewed as constructing buildings without the need for acquiring and preparing raw land. The buildings frames will be the “land.” At least theoretically, this presents a tremendous opportunity for savings, or at least an offset of some of the additional costs of building in freeway airspace.
Types of Costs:
Costs can be broken into three broad categories: soft, hard, and downstream. Cost categories can include the following:
|Soft Costs||Hard Costs||Downstream Costs|
|Environmental review||Acquisition / assembly of adjacent land||Maintenance of cap common areas and open space|
|Design||Cap structure construction||Programming|
|Fundraising||Improvements on cap (park, fixtures, buildings)||Energy (e.g., lighting, filtration equipment)|
|Legal||Traffic ramp improvements and realignments||Maintenance and repair of equipment (e.g., filtration)|
|Regulatory compliance||Utilities||Maintenance and repair of cap structural components.|
The costs leading up to construction will have the most variability of all the costs. However, of these costs, the architectural design and the engineering will be the most traditional and predictable of the soft costs. The Hollywood Central Park cap, a planned retrofit cap (i.e., not part of a larger freeway project) park project in Los Angeles is estimating their environmental review at between $1 million and $2 million.
Detailed data on highway decking projects is hard to come by. For example, the Park 101 District Freeway Cap Feasibility Study (August 2010, AECOM) notes “Many of the original and/or detailed cost documents associated with cap and park development were not available from the agencies charged with park construction and operation, nor from secondary sources.”
Given the similarity to bridge construction, at least regarding the load bearing structural elements, bridge construction costs give some guidance. A 2016 Arkansas guide gives a rough estimate for new bridges as $130 /s.f. but more in seismic areas. A California DOT guideline lists a range of costs for new bridges between $90 – $450 /s.f. The document identifies factors that increase or decrease the price range:
Factors for Lower end of Price Range: Short spans, low structure height, no environmental constraints, large project, no aesthetic issues, dry conditions, no bridge skew, urban location, seat abutment spread footing, no stage construction.
Factors for Higher end of Price Range: Long spans, high structure height, environmental constraints, small project, aesthetic issues, wet conditions (cofferdams required), skewed bridges, remote location, cantilever abutment, pile footing (large diameter piling), 2 stage construction.
Factors that will increase the price over the high end of the price range 25%-150%: Structures with more than 2 construction stages, unique substructure construction, [and] widenings less than 15 ft.” [capitalization modified]
– – It also notes items that are not included, such as: “. . . time related overhead, mobilization, bridge removal, approach slabs, slope paving, soundwalls, or retaining walls.”
A preliminary study in San Diego examined the cost of covering a stretch of the I-5 freeway dividing the city’s downtown from its famous Balboa Park. That section of freeway was already in a below grade trench that obviated the need for significant excavation. Additionally, there were existing bridges spanning nearly every block along that stretch. Conceptually, the study looked at decking the freeway between bridges at First Avenue through Sixth Avenue and described them as “Plazas” A –F. An additional Plaza between 7th and 8th, constituting 216,000 square feet was described as “Plaza G.” – seven plazas in total. It identified approximately 460,000 total square feet of potential deck. It included in its cost estimates: bridge construction, design engineering, construction engineering, and contingencies. It excluded: “upgrades to the existing facilities such as existing bridges, roads, or buildings.” The Plazas A through F were estimated to cost between $7.1 mil. and $12.3 mil each, and Plaza G to cost $51.6 mil, or roughly around $110 mil. total for all 7 plazas, or $240 per square feet. Costs for park development on Plazas A – F was around $1.1 mil. and $8.2 mil. for Plaza G. Building development ranged from $5.3 mil for Plaza D to $38.1 mil for Plaza G. However, a subsequent cost estimate by AECOM prepared in 2010 (Feb. 11, 2010) gives a cost estimate of $765 per square foot. or $518 mil. total. The AECOM estimate explained the substantial difference between it and the previous (Simon Wong) report, at least in part, as follows:
. . . freeway lid parks planned for Downtown San Diego are also likely to require freeway improvements when the lids are constructed. Caltrans estimates the approximate 2009 cost of these improvements to be $48,000,000 to $53,000,000 or $727 to $803 per square foot. For our preliminary cost estimate for your planning and discussion purposes, AECOM has used the mid-point within the per square foot range of cost identified in the Caltrans Supplemental PSR or $765 per square foot. Assuming the approximate combined area of 677,500 square feet for the eight lid parks in San Diego, we estimate their total cost to be approximately $518 million.
A recent study on a deck park for I-94 in San Diego puts a cost range on the unadorned deck structure at between $400 – 750 per square foot.
A 2013 presentation prepared for the City of Glendale regarding its Space 134 freeway cap concept gave cost estimates for other contemplated but unbuilt cap projects on the west coast ranging from $218 to $551 per square foot. It is presumed that since the cost estimates were listed without limitation to a particular phase or aspect, the estimates include all capital costs but not downstream costs. The project costs were as follows:
- Park 101: $551 per square foot for a 34 acre project or $413 per square foot for a 22 acre project (no explanation was given for the reverse economy of scale but likely the smaller project excluded construction of the most difficult and expensive section).
- Hollywood Central Park: $505 per square foot for a 44 acre cap (now reduced to 38 acres).
- Santa Monica Cap: $399 per square foot for a 5 acre cap.
- Olympic Sculpture Park, Seattle: $218 per square foot for a 9 acre cap.
One strategy for reducing costs is to avoid or defer the deck construction that require modifications to existing on or off ramps. On the other hand, the Park 101 Feasibility Study noted the need to:
“accommodate future widening of the freeway should Caltrans or another transportation agency deem it necessary. This is accomplished by a support system placed at a distance far enough apart to allow lane expansions without impacting the pylons supporting the cap structure itself.” (Ibid. at 4-3)
Where deck-top structures are contemplated, some cost synergy may arise from integrated construction opportunities, particularly in the shared material and coordinated construction of the bridge structure and the foundations of the buildings.
Thus, cost estimates are widely disparate, with estimates ranging from $200 per square foot to $750 per square foot. Costs are highly dependent on regional and project variables.
 See e.g., City of Glendale, Space 134 presentation, (2013) – modified from original.
 Arkansas Department of Transportation, Estimated Costs per Mile, 2016, Retrieved from https://www.arkansashighways.com/roadway_design_division/Cost%20per%20Mile%20(JULY%202016).pdf (accessed Sept. 29, 2017)
 California Dept. of Transportation, Div. of Engineering, Comparative Bridge Costs (January 2015), retrieved from http://www.dot.ca.gov/hq/esc/estimates/COMP_BR_COSTS_2014-eng.pdf (Sept. 29, 2017)
 The I-5 Downtown Deck Study Preliminary Study Report (August 26, 2003 by Simon Wong Engineering for Centre City Development Corporation – the city’s redevelopment agency aka CCDC).
 AECOM cost estimate prepared for CCDC in 2010 (Feb. 11, 2010).
 CalTrans, Freeway Cap Best Practices & SR 94 Cap Park Study (November 2016), SR 94 Freeway Cap Park Feasibility – Planning-Level Capital Cost Estimates and Planning-Level Operations & Maintenance Cost Estimate, pp.48-54, Retrieved on Oct. 16, 2017 from http://www.dot.ca.gov/dist11/departments/planning/planningpages/capstudy.htm
 City of Glendale, Space 134 presentation, (2013) – acre cost estimates converted to square feet estimates by author. All costs estimates were for projects still in planning stages.
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