As Palo Alto prepares to adopt a new law banning idling vehicles, city officials are warning that the new prohibition -- while broadly popular among the City Council -- is unlikely to see any real enforcement.

If approved, the law would make it illegal for drivers to run their engines without actually going anywhere for more than three minutes. The new law was pitched by activists from a local Sierra Club chapter earlier this year and later embraced by the City Council, with four council members authoring a memo in August that made a case for an anti-idling ordinance.

In advocating for the ban, council members and activists alike pointed to the health and environmental impacts of exhaust fumes emitted from stationary vehicles. The memo from Vice Mayor Liz Kniss and council members Eric Filseth, Karen Holman and Tom DuBois, also argues that there is no good reason for extended curbside idling.

The council members estimated that idling vehicles produce about 6.2 tons of greenhouse-gas emissions per year, with tour buses, tech shuttles, delivery vehicles and construction vehicles identified as some of the top culprits.

"A city ordinance requiring drivers to shut off their engines after the more feasible (time) of two or three minutes of stationary idling would make a modest but measurable contribution to our 80-30 goal and improve health conditions at a low cost," the memo states. The 80-30 goal calls for the city reduce emissions by 80 percent come 2030.

The council endorsed the memo by an 8-0 vote, paving the way for the creation of the new ordinance that will be based on similar efforts in Ann Arbor, Michigan; Salt Lake City, Utah; and Minneapolis, Minnesota. On Tuesday night, the council's Policy and Services Committee is set to vet it before it goes to the full council for final approval.

But even as the committee prepares to sign off on a law that makes it illegal for drivers to idle for more than three minutes (barring special circumstances), city staff are warning that acknowledging that formal enforcement activity will be "challenging and of limited effectiveness," according to a new report from the office of City Manager James Keene.

"Code enforcement and police officers will rarely be available to respond to complaints of violations due to limited resources and the need to prioritize other health and safety issues," the report states. "And even when resources are available, violations typically would not be ongoing when enforcement officers arrive at the scene, making formal enforcement activity difficult."

Given these challenges, one of the questions that the committee will ponder is: Should the law include an enforcement mechanism at all? Or should the ordinance be thought of as an education tool -- a mechanism for nudging rather than punishing?

To date, staff has been agonistic on this question. The draft ordinance, which the city released last week, includes four possible options for dealing with the enforcement conundrum. One approach is to simply omit an enforcement section entirely. Another is to include an enforcement mechanism but specify that the city will use it until the City Council adopts a formal resolution requiring it to do so.

The other two options take more putative approaches. One would set a $100 fine for the first violation of the anti-idling ordinance, which would go up to $150 for a second and $200 for a third within the same calendar year. The most stringent alternative would allow the city to issue administrative or criminal citations for idling, with fines starting at $300 for the first violation and then going to $450 for a second one and $600 for a third.

The law includes about a dozen exceptions. These include instances where a vehicle has to stay still because of traffic congestion or traffic signals; when the driver has to operate a defroster, heater or air conditioner to prevent a health emergency; when the temperature is cold enough (below 40 degrees) or high enough (above 85 degrees) to warrant heat or cooling; or when a vehicle has mechanical problems.

The law also makes exceptions for emergency-response vehicles, private-security providers and armored vehicles that idle while in the course of their business.

The enforcement challenge is unlikely to deter the council from pursuing the anti-idling law. In August, several council members argued that the campaign would be worthwhile even without enforcement. The memo proposes installing signs in "idle-rich places" such as schools, truck stops and employee bus stops. It also suggests a "community education and outreach" campaign to raise awareness.

During the August discussion, Filseth said the proposal has "very little dependency on enforcement."

"It's just getting people to think differently -- like separating our food scraps," Filseth said.