Holloway Green Street San Francisco

Completed in 2017 and funded with $7.4 million through the Clean Water State Revolving Fund, the Holloway Green Street was the first use of pervious concrete in a public right-of-way in San Francisco. The brief was a stormwater brief: capture rainfall on the street, keep it out of an ageing combined sewer system, and return it toward the Lake Merced watershed. By the SFPUC's own monitoring, it delivered — a 77% reduction in stormwater volume entering the sewer in year one, 78% in year two.

The design routes street runoff through a treatment train, and it's worth understanding the sequence before judging the outcome.Rain falling on the porous concrete parking lanes infiltrates straight through the surface rather than sheeting off — pervious concrete carries a high void ratio that gives it a strong initial infiltration rate, sending water into an aggregate storage layer below instead of into the gutter. Runoff from the sealed traffic lanes is handled separately: kerb inlets and flush kerb cutouts at the intersection bulb-outs divert it sideways into bioretention rain gardens, where engineered filter media and native planting slow, filter, and infiltrate it. What the street doesn't pass to the sewer, it returns to the watershed. That sequence — infiltrate where you can, divert and treat the rest — is what the 77–78% figure represents.

The installation is holding up well. We saw no pavement failures, no cracking, no significant structural deterioration. The signage is excellent — clear, educational, well designed; corner boards explain the catchment, the watershed history, and how the integrated system functions. Someone put real care into telling this story to the neighbourhood.

It is, by any fair measure, a serious and successful piece of stormwater infrastructure. It has become a reference project for municipal pervious concrete across the region, and the SFPUC deserves credit for building it, monitoring it, and publishing the data.

Standing in the middle of Holloway Avenue on a sunny morning, there's almost no shade. There are street trees — perhaps ten across the full eight blocks — but every one of them pre-dates the project. The Green Street planted no new trees, and connected none of the existing ones to the stormwater system it built around them. The canopy that's there is a leftover, not a design decision.

The rain gardens in the bulb-outs carry native plantings at ground level — some well established, some still sparse — and they add real greenery to the streetscape. But ground-level planting and canopy are not the same thing. Holloway is a bicycle corridor linking Balboa Park BART to San Francisco State University; students, commuters and residents use it every day. On the day we visited the footpath was exposed for almost its entire length — the handful of mature trees, spaced across eight blocks and never part of the water design, do little for a corridor this long. Little overhead relief, almost no cooling canopy — just pavement, painted kerbs, and low groundcover, and a few trees the project left untouched.

The systems are also maintenance-dependent in ways worth naming. Pervious concrete clogs as fines fill the surface voids, so it needs periodic vacuum sweeping to hold its infiltration rate; some panels showed staining consistent with early clogging, and an infiltration test would confirm whether rates have dropped. The bioretention beds need regular weeding — we spoke with a crew hand-weeding the day we visited. The investment, capital and operational, is substantial. Which makes the absence of canopy not just an aesthetic gap, but a missed return on ground that was already dug, drained, and paid for.

This is not a Holloway problem. It is a pattern across the industry.

The brief that governs most green-street projects is written by water engineers and stormwater planners — and water engineers solve water problems. Trees, when they appear at all, are an afterthought: a landscape line item considered after the drainage design is locked, and often the first thing cut when budgets tighten or subgrade conflicts arise. The result is streets that perform hydrologically and fail humanly — streets that manage rainfall but bake their residents.

There's also a hard-won engineering reason for the caution. Street trees in paved environments routinely fail: roots restricted by compacted subgrade can't reach the volume they need, trees decline, heave pavement, and get removed. Engineers who have watched that happen learn not to specify them. The instinct is rational. It's the assumption underneath it — that a tree can survive in a small pit of compacted soil beneath a hard surface — that need srevisiting.

Shade and stormwater are not competing objectives. They are complementary — and the place they meet is the soil profile under the pavement.

A mature canopy reduces surface temperatures by intercepting solar radiation before it reaches the pavement, cuts the urban heat island effect, and intercepts rainfall at source — research consistently shows canopy cover reduces stormwater runoff volumes before water reaches a drain. It does this for decades, compounding as the tree grows.

Delivering that canopy on a hard-paved street is an engineering problem with an engineering answer. Suspended-pavement and structural soil-cell systems provide the rooting volume a large tree actually needs — typically in the order of 15 to 30 cubic metres of uncompacted soil — inside the same footprint as the surface infrastructure, protected from compaction and surface loads. Critically, that same engineered soil profile can be designed into the treatment train: detaining, filtering and slowly releasing stormwater, so the tree pit functions as bioretention in its own right.

Green infrastructure that doesn't grow tall enough to cast a shadow is only doing half the job.

The most useful change isn't a product or a detail — it's when the question get sasked. The next generation of these projects needs a different brief at the outset: not only "how do we manage the water?" but "how do we make this street liveable?" The answer to the second almost always involves trees, and trees, planted in the right conditions with adequate soil volume, answer a good part of the first. The tools exist. The knowledge exists. What needs to change is the order of operations — bringing canopy and water into the same conversation before the drainage design is set.

Holloway is a genuine achievement, and a generous teacher. It shows how well the water side of the discipline now performs — and exactly where the next gain is waiting.