From a health and environmental standpoint, the most important result of street sweeping is the debris you cannot see.
Recent research has revealed that a chemical found in car tire debris kills coho salmon returning to spawn. For the first time in years, endangered Coho salmon have been spotted in tributaries of San Francisco Bay.
High levels of organic and inorganic contaminants in street dust represent a source of dual potential risk to stormwater and air quality. Further, studies have indicated that as much as 85 % of ambient airborne particulate matter (PM 10), exposure to which is associated with several adverse health effects, can arise from accumulated street dust.

The AB 617-driven Community Emissions Reduction Plan is providing surprising and compelling evidence that marine and rail operations, along with vehicles and trucks, are the source of most of the hazardous PM 2.5 Parked cars pose a significant interference to street sweeping.

Parked car interference can have a significant negative effect on the ability of street sweepers to pick up accumulated particulate material. Since it is reasonable to assume that all street dirt accumulation essentially occurs within the width of a parked car, access to the curb is denied for the entire length of the parked cars along with the additional distance it takes for the sweeper to maneuver around parked cars. The most skilled sweeper operators can minimize this additional interference to a distance of approximately the length of a single car on both sides of one or more parked cars. So a good program minimizes parked car interference by sweeping at night in commercial or industrial areas and during the day for residential areas. It also uses and enforces residential parking restrictions where they are warranted.
This is counterintuitive from the widely held belief that Chevron is the biggest threat. While Chevron remains a significant source of PM2.5 particulates, its impact on health is relatively small compared to other sources.

While Chevron emissions as a source of PM2.5  far exceed that of road dust, the impact of road dust on human health is far higher. Source: Richmond – North Richmond – San Pablo Community Path to Clean Air, Community Emissions Reduction Plan (CERP) Community Steering Committee Meeting #154, May 16, 2022, presentation. (http://www.tombutt.com/pdf/path%20to%20clean%20air.pdf)

As a source contribution to cancer risk, Vehicles and Trucks (including road dust) and marine and dominate, far exceeding that of Chevron. Source: Richmond – North Richmond – San Pablo Community Path to Clean Air, Community Emissions Reduction Plan (CERP) Community Steering Committee Meeting #154, May 16, 2022, presentation. (http://www.tombutt.com/pdf/path%20to%20clean%20air.pdf)

Of the vehicle and truck sources of PM2.5, road dust makes up 68 percent. Source: Richmond – North Richmond – San Pablo Community Path to Clean Air, Community Emissions Reduction Plan (CERP) Community Steering Committee Meeting #154, May 16, 2022, presentation. (http://www.tombutt.com/pdf/path%20to%20clean%20air.pdf)

Of the Vehicle and Trucks component, road dust contributes 68 percent of the total PM 2.5 particulates.

Road dust represents a growing portion of on-road emissions inventories due to recent reductions in vehicle exhaust emissions; this category is currently the subject of a study by CARB, Caltrans and EPA.[1]
One of the recommended strategies for reduction is street sweeping:

Control road dusts trough street sweeping and/or reducing trackout from construction projects and industrial sites.

Street sweeping is recommended as a way to control road dust. Source: Richmond – North Richmond – San Pablo Community Path to Clean Air, Community Emissions Reduction Plan (CERP) Community Steering Committee Meeting #154, May 16, 2022, presentation. (http://www.tombutt.com/pdf/path%20to%20clean%20air.pdf)

In the last 20 years, the Richmond City Council has continued to waffle over street cleaning policy, pondering whether moving vehicles on sweeping days should be mandatory or voluntary. Most of the City’s neighborhoods have been signed for years with violators at least threatened with citations and/or towing.

There is a clear resentment from neighborhoods that have embraced street sweeping against those neighborhoods that have arm wrestled certain City Council members to exempt them from mandatory compliance for moving vehicles on street sweeping days,
Although there are at least two neighborhoods without signs (Richmore Village and Carriage Hills South) that appear to generally respect the requirement to move vehicles, there are only two other neighborhoods (Richmond Annex and Annex Panhandle) that are generally not in compliance and continue to resist signs and citations.

Whether street sweeping is legally required, and further, whether signs and citations are legally required by regulatory agencies enforcing the Clean Water Act remains murky.
What is becoming increasingly clear, however, is that street sweeping is a critical component of improving human health and keeping pollutants out of creeks and San Francisco Bay. Among the best practices of an effective streety sweeping program is the motivation provided by signs and citations to move vehicles on sweeping days.

It is ironic that members of the City Council who hold themselves up as protectors of the environment are leading the charge to make vehicle removal on sweeping days voluntary, reducing the effectiveness of the program, increasing pollution of waterways and San Francisco Bay and posing a significant human health hazard, all to simply cater to the convenience of residents in a few selected neighborhoods.