Sure, newly planted shrubs and trees can soak up storm water that otherwise would have overwhelmed the sewage system.

Now there is early evidence that they may improve health and safety as well.

Four years ago, the Philadelphia Water Department launched an innovative - and, at a projected cost of $2.4 billion, expansive - storm-water plan.

The goal is to pepper the city with myriad small projects - from rain gardens to manufactured wetlands to green roofs - that will sop up the first inch of rain. When all the projects are completed two decades from now, no longer will storm water inundate the city's sewers and gush untreated into streams, carrying road oil, raw sewage, and other pollutants.

The impetus was environmental, and cost was a key reason for the design. These days, however, when Water Department officials tout the benefits of the plan, they often refer to the "triple bottom line," saying it has social benefits as well.

Going green is believed to be far less expensive than the huge underground tunnels that other old cities are building to temporarily hold the water. Some are miles long, as big as a subway tunnel.

The economic and environmental benefits were quantifiable. Far squishier were the social benefits. Some research had suggested creating parks or cleaning up vacant lots might be linked to lower crime rates and better health among nearby residents. But it wasn't clear that green storm-water projects would offer the same benefits.

This is a big deal. An emerging form of urban greening is storm-water infrastructure. Philadelphia's concept has been touted as the biggest and most ambitious in the nation. Other cities are watching.

So is Michelle Kondo, formerly a postdoctoral student at the University of Pennsylvania's Center for Clinical Epidemiology and Biostatistics and now a scientist with the U.S. Department of Agriculture Forest Service in Philadelphia. A few years ago, Kondo began to study 52 of the city's green storm-water projects. All had been built before 2013, as the city was first implementing its plan. Most were very small.

"Some of them are just two feet by five feet - planters or little rain gardens," Kondo said. "But they also tend to be in public spaces."

She and her colleagues, including Penn epidemiologist Charles Branas, who had studied vacant lots, compared the 52 sites with 186 similar areas where projects were planned but not yet built. They factored in socioeconomic data from the census, crime statistics, and health data.

Their results, published in January in the American Journal of Public Health, were an eyebrow-raiser. Between 2000 and 2012, incidents of drug possession at the project sites dropped by as much as 27 percent compared with the control sites.

As far as a half-mile away, "we saw a significant reduction," Kondo said.

She spoke to police, who interpreted the findings as roughly parallel to the "broken-windows" model of policing - clean up a community by enforcing laws against vandalism and other petty crimes, disorder will decline, and residents will take more responsibility for their neighborhood. Storm-water projects - living patches of green that are regularly maintained - may signal what the study refers to as a community's "defensible space": areas the neighborhood is engaged with and watches over.

Christopher Crockett, the Water Department's deputy commissioner of planning and environmental services, praised the study's vigor and said it showed "we're on the right track."

With the tunnels that other cities have embraced, "all you do is have negative impacts on communities," Crockett said. "You dig holes in the ground, you tear up neighborhoods for decades, and the benefits of reduced overflows occur someplace else, near the streams."

With incremental greening projects, some benefits kick in right away, right there.

What the study did not find, however, remains puzzling. The researchers saw no significant reduction in nondrug crimes, such as assaults or burglaries. Nor did they establish a link to measures of better health among nearby residents, such as lower blood pressure or cholesterol levels.

That may come. The Water Department is still ramping up. Hundreds more projects have been completed or are in the works. Future studies will have more robust data.

Kondo wonders: Are some types of projects better than others? Does it matter if they're at the end of a block or in the middle? Do multiple projects in close proximity produce benefits that overlap and build on one another?

The study concluded that although health and safety outcomes are not typically part of cost-benefit assessments of green storm-water projects, they should be.

Philadelphia planners made an early stab at it, initially calculating that every dollar put into the storm-water project would return a dollar of combined economic, environmental, and social benefits. It was a rough estimate - and, officials believed, a conservative one.

Now they have a little more to go on.

"GreenSpace," about the environment and health, appears every other week, alternating with Art Carey's "Well Being" column.