Planners at San Francisco’s Municipal Transportation Agency are putting the finishing touches on a new citywide parking meter rate system, one that will automatically hike or drop rates in popular neighborhoods depending on availability of spaces.

Rates would be adjusted every three months and could be as high as $8 an hour in high-volume areas, or as low as 50 cents an hour where parking is underused. The rates would affect both street and garage parking. If parking is scarce, the price would go up 25 cents, with $8 the top rate.

It’s called “demand-responsive” pricing and operates under the premise that the higher the meter rates, the quicker people will free up spaces.

“We just need one or two people to make the decision to park on a different block to impact parking availability,” said MTA spokesman Paul Rose. “People can also park at a different time or take a different mode of transportation.”

Rose said pilot programs found that rates dropped far more often than rates went up.

San Francisco Council of District Merchants President Henry Karnilowicz said he’s puzzled by the MTA’s logic.

“I don’t see how increasing rates is going to make the turnover any different,” Karnilowicz said. “People will just have to pay more. If you want to increase turnover, why not just put time limits on parking?” — like the 24-minute or one-hour meters already around the city.

Rose said that time limits would be too prohibitive.

“People could be forced to cut short their meal or shopping,” he said.

Rose cited a pilot program MTA ran in the Marina, the Fillmore, downtown and other neighborhoods last year that showed an increase in parking turnover and more business for shops when the prices were based on hourly use.

However, Marina Deli owner John Nazzal, whose Chestnut Street store was in one of the test areas, said that though parking may have been freed up, the cost was too high.

“You don’t want to spend $4 in parking to get a $9 sandwich,” Nazzal said.

“Anytime you talk about parking, you are going to get some resistance,” Rose said.

And one of the first weighing in is Supervisor Jeff Sheehy, who blasted the plan as a financial hit on already stretched middle- and working-class families.

“The only ones who will benefit will be ride shares like Lyft and Uber, who will become even more attractive to use,” Sheehy said.

But SFMTA board member Art Torres said the intent was to keep the overall rates flat.

“We have to see if that works out,” Torres said.

The plan goes to the SFMTA board on Oct. 17. If approved, the new rates could go into effect later this year.

Traffic stop: There was more to the Golden Gate Bridge district’s decision to end the practice of allowing the roadway to be used for special events than just the safety of marathon runners.

The same “special events” language that would have allowed the roadway to be temporarily closed for runners could also be used by political groups who wanted to use the bridge for marches or other protests.

Even allowing for a “special exception” like the very popular San Francisco Marathon could have left the district open to a legal challenge.

“You can’t allow a waiver for one group to close lanes to traffic and then deny a waiver to another group like Patriot Prayer, who met the same criteria,” said bridge district General Manager Denis Mulligan.

Patriot Prayer is the conservative “free speech” group that tied San Francisco officials in knots when it received a permit to demonstrate at Crissy Field.

A march across the bridge would come under the umbrella of “expressive activity” covered by the First Amendment and leave the district with no choice but to issue a permit if the group could come up with the cost of erecting a temporary traffic barrier.

In the end, the bridge board’s 10-4 vote dictated that the main job of the bridge is not special events but to carry cars and trucks over the Golden Gate, 24/7/365.

San Francisco Chronicle columnists Phillip Matier and Andrew Ross appear Sundays, Mondays and Wednesdays. Matier can be seen on the KPIX TV morning and evening news. He can also be heard on KCBS radio Monday through Friday at 7:50 a.m. and 5:50 p.m. Got a tip? Call (415) 777-8815, or email matierandross@sfchronicle.com. Twitter: @matierandross