Tucson, Arizona, Green Streets Policy
The Tucson Green Streets Policy requires new and reconstructed roadways be designed to harvest the first 1/2 inch (13 mm) of rainfall in order to grow streetside and median vegetation. The policy is technically the Tucson Department of Transportation’s “Active Practice Guidelines.”
This is a dramatic 180º shift in how our streets and adjoining rights-of-ways are designed and built. In the past, lifeless grey infrastructure of asphalt and concrete drained nearly 100% of the rainfall, while generating higher summer temperatures due to the heat-island effect from all the exposed hardscape. Now living green infrastructure is being built and planted along and within the streets to harvest and reinvest nearly all the rainfall and street runoff from a typical storm to generate and grow living air conditioners and water/air/soil filters of beautiful, shading vegetation. Note: The next time this policy is reviewed, I’d like to see if it would be possible to revise the Performance Goal (under section D) which states that “Green Infrastructure basins are designed to accept a maximum final pooling depth of 8 inches,” to allow for deeper, higher-capacity pooling depths to increase capacity.
For a Tucson, AZ, study looking at the cost-to-benefit ratio of retrofitting existing infrastructure with green infrastructure, see Solving Flooding Challenges with Green Stormwater Infrastructure in the Airport Wash Area. This study utilizes a Holistic Cost Benefit analysis to assess a full range of benefits that would be achieved with a Green Stormwater Infrastructure (GSI) retrofit of residential properties, rights-of-way (ROW), and streets, as well as commercial properties, schools, churches, other community nodes, and their associated parking lots within three built-out watersheds in the south of Tucson. Results from modeling show GSI can have a significant impact on both large and small storm events. GSI resulted in reducing the 100-year 3-hour event peaks in the three watersheds by 24%, 19% and 10%. GSI implemented throughout these watersheds in the 25-year scenario of the study would result in over $2.5 million dollars of annual community benefits as a result of flood reductions, water conservation, property value increases, reduced urban heat island impacts, improved stormwater quality, reduced heating and cooling needs, air quality improvements, and the energy associated with pumping CAP water and groundwater in Tucson. Residential rain gardens, curb-cut rain gardens, and Green Street features have benefit/cost ratios greater than 1 showing that benefits outweigh the costs by 4.4, 2.7, and 2.1 for those features respectively.