We need to foster innovation in Complete Streets design by challenging assumptions

By: Mark Reiner, PhD, PE, and Steve Brooks, AIA, LEEDap

This blog builds on comments from the authors to the general guidance regarding utilities in Denver Complete Streets Design Guidelines – Final Draft 073120 (The Guide)Denver Department of Transportation and Infrastructure's (DOTI) 112-page Complete Streets Design. The response requested was, understandably, limited to – what did you like? and what would you add? Denver’s Department of Transportation and Infrastructure presented a thoughtful timely approach to remaking Denver’s streets and prepare for a better post-pandemic world. These ideal prototypes will assist in getting consensus on desirable results. The philosophical approach for complete streets with increased activity, pedestrian, cycling and transit areas (with the auto as a guest) is excellent. However, by presenting the general guidance as to how utilities are typically placed below our urban roads, we are poised to continue the same paradigm that has led to our aging urban infrastructure crisis. Innovation requires that assumptions be revisited. This short blog looks at four assumptions that should be challenged on page 97 of the Guide.

What if the scope of work for urban planners included the task of classifying areas that would be extremely disruptive if failed infrastructure caused road closures and property damage? They would need uniform classifications to facilitate the ranking of importance that certain residential, economic or commuter corridors are for maintaining normal city life. Rather, current urban planning either adopts custom city characterizations, or most often, use the engineer’s language of classifying corridors by road types: e.g. arterial, collector, local, highway, 2x2, etc. Emergency and evacuation routes are identified simply as – emergency or evacuation routes. But these routes are not evaluated for the potential of disruption that underlying infrastructure could pose (think arterials and bridges closed or failed). We look at four assumptions from page 97 of the Guide.

Utilities should be placed to minimize disruption

The schematic shown on Page 97 shows an extremely important factor in street configuration – the underlaying infrastructure. In addition to creating complete streets above grade, the new urban road paradigm must also be changed to address the looming problem of aging infrastructure. Urban life is perpetually disrupted by road construction and the repair and replacement of underground infrastructure, which are chronic hazards to urban resilience. New York City, for example experiences 550 road cuts per day and suffers $300 million per year in just direct damage costs (excluding failures). It has been estimated that indirect costs of these disruptions may be 29 times more than the direct costs that arepaid by the utility.

We should take a 3D approach to how we design urban roads to account for the much broader spectrum of life-cycle costs of infrastructure. Urban planners can define what areas of the city should not be disrupted. City works department should have input into the comprehensive design process to determine what surface conditions are best for access to utilities and in new construction, where utilities should be located.

Utilidors and reconsidering right-of-way requirements

The schematic assumes that infrastructure below our urban roads are buried in a soil matrix. This assumption results in adopting vertical spacing assumptions of buried utilities buried below our urban roads. This has led to a conception that there is a State requirement for minimum separations of 10’ from outside of pipe between water, sewer and storm. Resulting in a typical street with a 12” sewer, 24” storm and 12” water, that is 24’ wide. Assuming 1’-2’ clear from outside of utilities many streets are designed to be 28’-30’ wide. But these separations are design criteria developed by the Colorado Department of Public Health and the Environment (CDPHE) – not regulations.

Where urban planners have determined areas of a city that should not be disturbed by constant road maintenance [1, and 2], utilities could be placed in wet and dry utility corridors (aka, ‘utilidors’) to create ‘disruption-free zones’ for city life. Utilidors change the matrix that surrounds our utilities from soil to air. Reducing all right-of-way requirements (with the exception of emergency vehicles that may trump the horizontal requirements). And, rather than providing access to utilities via ‘vaults’ located in the parking lane that access only a tiny fraction of the buried assets, utility tunnels allow full access for repair, maintenance, and replacement for all utilities – without disturbing the city’s socioeconomic engines that depend on reliable urban roads. Utility tunnels have been estimated to reduce infrastructure asset failures by approximately 80-95%, extend asset life by approximately 15-30% and remove the need for repeated excavation and repaving procedures over its lifetime of 60-100 years, saving significant long term and indirect costs.

The hazards of collocation

The Guide takes the approach that utilities are a nuisance to a street scape rather than being critical to the vitality of city life. Roads and utilities have to be viewed in a single paradigm [1] that is critical to urban vitality. Rather than stating that - “Utilities should be consolidated where feasible to maximize efficiency and minimize disruption to streetscape elements. Locating utilities underground whenever possible”, the current theme of sustainable infrastructure planning should be “approach urban infrastructure as a series of systems that should function in synergy and be directly linked with urban planning.” [3].

A methodology is required to better understand the hazards to streetscapes from collocating our buried infrastructure; and the significance in difference between immediate physical failures of infrastructure (e.g., water main breaks) vs cascading operational failures (e.g., loss of power results in potable water not being delivered to customers). Physical failures result in our pavements being cut for immediate repairs.

Forever pavements

This blog is also not intended to show schematics of tunnels that separate wet from dry sectors, nor discuss the materials and costs that would lead to ultra-longevity roads. Rather, we use asphalt for our roads as a material that can be cut into with ease and placed quickly over the buried utilities. And, due to the differential settlement of the soils, and freeze/thaw cycles, our norm/assumption is that our roads will soon have potholes and cracks that allow water into the subsurface to further exacerbate subsurface conditions. However, what if the cross-section of a ‘complete street’ on page 97 contained all infrastructure in utilidors designed for HS-20 loading. The research and materials, and perhaps life-cycle costing, exist to construct roads that never have potholes and have no need cutting to gain infrastructure access. Wouldn’t this be the foundation of a smart city?

The authors have been involved with the implementation of such utilidors in a new smart green city in Kenya, placed in the median of main boulevards for ease of access. Although this location inhibits the placement of large trees in the medians- shrubs, smart poles with efficient streetlight standards can occupy this space without the shadowing interference of shade trees, which are placed in the curbside amenity zone over pedestrians.

Footnotes

[1] See five-part series starting with ‘From Appian Way to Modern Decaying Infrastructure’

[2] See four-part series starting with ‘Communication Breakdown – City vs Asset Management’

[3] Pollalis, Spiro N. Planning Sustainable Cities: An infrastructure-based approach, 1st Edition 2018 Routledge, 20160520. VitalBook file.