Hawaii enacted a law in 2006 that bans smoking in indoor and partially enclosed public spaces, such as restaurants and bars as well as office buildings and workplaces. The ban has since been expanded to include vaping with electronic smoking devices.

The public health case for going smoke-fee is compelling: Eliminate health dangers tied to secondhand smoke, reduce the environmental impact of cigarette butts and keep impressionable children from picking up a bad habit.

However, a Honolulu City Council proposal that would expand the ban to include the private sphere of one’s vehicle is a well-meaning misstep.

Bill 70, approved on first reading Wednesday and headed for Council committee review, would prohibit smoking tobacco or vaping in vehicles carrying children. While no child should be seated in a smoke-filled vehicle, of course, a city law is not a fitting fix.

Such a ban is an overprotective overreach in that it seeks to control a non-criminal behavior within the private space of one’s own vehicle. Smoking is legal for anyone age 21 and older in Hawaii as long as the puffing takes place in an area not deemed a public setting.

Such a ban prompts slippery- slope possibilities that could make our own homes the jurisdiction of politicians aiming to improve us — and consequently interfering unduly with matters of personal choice. Should smoking be banned in homes, too? Not unless cigarettes and e-cigs become illegal.

Valid city ordinances pertaining to motorists tend to be safety-focused. For example, text messaging while driving is illegal in Hawaii because drivers distracted by their devices are a rising cause of crashes.

In Hawaii, a health-focused law should involve the state Department of Health’s oversight. But proposals for a statewide ban on smoking in a vehicle with a child present have been unsuccessful. This year, Senate Bill 261 was approved by the Senate but then failed to secure a committee hearing in the House. That proposal tasked DOH with reporting on the ban in tandem with law enforcement agencies. Among the enforceability questions: How to handle a driver smoking with the window rolled down?

Nationally, eight states outlaw smoking while driving with a child in the vehicle (with varying ages for the child), while various counties and cities have similar laws. Hawaii County has had a ban on its books since 2010, and in July 2016 Kauai County voted 6-1 for a ban that sets the child’s age at 13 and younger. Kauai Council members lowered it from 18 to avoid placing a burden on police officers who might have difficulty gauging the age of older teens.

Kauai Councilman Arryl Kaneshiro, who cast the lone dissenting vote, rightly refrained from supporting legislation of “common sense items,” as he put it. “We could probably sit around all day making laws protecting us from ourselves,” Kaneshiro mused.

In decades past, when bans in public places were taking shape, they were met with resistance from people who had long lived in smoke-filled environments. But when the air cleared and scientific research proved that secondhand smoke — containing about 70 known or probable carcinogens — can cause heart disease and lung cancer among other health problems, common sense gradually prevailed in public settings.

Private settings — the last go-to for the nicotine-addicted — are another matter. Despite the dictates of common sense, some motorists are still lighting up in kid-filled cars in which the concentration of toxins can be several times greater than that of a smoky bar. And that’s plain wrong. But rather than institute a nanny-state ordinance, Honolulu should mount an anti-smoking campaign that underscores the terrible truth: More than 2.5 million nonsmokers have died from health problems caused by secondhand smoke over the span of, roughly, the past five decades.