Cities around the world use street sweeping to keep dirt and debris out of the air and water. Denver started doing the same more than a century ago.
Along with keeping roadways clean (or clean-ish, anyway), street sweeping produces a small but mighty source of revenue. Last year, Denver took in $6.9 million from the 138,895 parking tickets it issued to unfortunate souls who forgot to move their cars to make way for the sweepers.
For context, here's what that amount can and cannot cover in Denver:
• It would more than cover Mayor Michael Hancock's $5 million allotment to the Affordable Housing Fund in his proposed 2017 budget, which the city council approved in November.
• It would nearly triple money set aside to fix or install new sidewalks on city-owned property, like parks. About $2.5 million was earmarked for such projects in Hancock's budget.
• It would get us on a serious path to recycling and composting. Hancock's budget allotted $2.2 million to add four additional composting routes and three additional recycling routes.
• But it would be about $12 million short of the $19 million Hancock requested to "pave, repair, maintain and repave" roads and sidewalks around Denver.
The money collected from the tickets goes into the city's general fund. But money isn't the point of street sweeping. "The purpose of the residential street-sweeping program isn't to issue tickets," says Heather Burke, a spokeswoman for the Denver Department of Public Works. To that end, the city offers a service through denvergov.org/pocketgov/#/ that lets parkers set up a profile and get text and/or e-mail reminders about when they should move their cars.
But do those giant scrubbers just move dirt and trash around? Are our waterways and our air cleaner? Is it all worth it?
Actually, yes, says Michael K. Stenstrom, a distinguished professor at UCLA's Civil and Environmental Engineering Department. In the early and mid-2000s, Stenstrom and a colleague analyzed about twenty studies that claimed street sweeping wasn't an effective way to keep nasty stuff out of waterways. "I'd heard that a number of times, and I thought to myself, 'That can't be true,'" he says. "If you go down and look at what street sweepers pick up at the end of their shifts, you see all kinds of stuff stuck in there."
Out of the twenty studies, Stenstrom and his colleague found that sixteen had what statisticians call a "type 2" error. "Essentially, there weren't enough data points to find a significant difference in water quality due to street sweeping, even if one existed," he explains. Of the four studies that did have enough data, three "found a significant improvement in water quality for street sweeping."
Street sweeping not only removes large pieces of debris, like bottles and cans, but also lead and other metals, 80 percent of which don't dissolve in stormwater but instead cling to particles like sand and soil (think of construction sites) and tire tread. "The improvement is not a 95 percent reduction in pollutants, but it is a statistically significant reduction," Stenstrom notes.
Last year, Denver's street sweepers swept 143,112 miles and collected 62,848 cubic yards of dirt and debris, or about enough to fill 5,237 dump trucks. Those figures would undoubtedly be higher if we could all just remember to move our damn cars.