SAN DIEGO — San Diegans could be taxed for every mile they drive if a new proposal by local policymakers is approved — an idea that’s prompting fierce debate.

The potential new tax is being considered by the San Diego Association of Governments, a collection of county agencies that make decisions about metro planning. Representatives say it’s all part of an ambitious $163 billion dollar plan to reimagine transportation around the region.

The policy would introduce a 2- to 6-cent tax on drivers per mile on local roads to help pay for the improvements. SANDAG is still looking into how it would track a driver’s miles for the tax but it may use transponders, smartphones or odometers, leaders say.

Proponents say the tax would help fund the county’s step into a more efficient, environmentally friendly future. The 30-year plan would slash greenhouse gas emissions and introduce everything from new public transit hubs to freeway improvements that ease commutes, SANDAG says. Officials also say it will give a helping hand to low-income residents whose lack of transportation options limits their opportunities.

“We’re talking about a big picture, regional transportation plan that looks out to the year 2050,” said Coleen Clementson of SANDAG. “We’re talking about a $163 billion dollar investment for the San Diego region that really once and for all addresses traffic congestion, address social equity in a meaningful way and meets some really aggressive climate targets that have been set by the state of California.”

Detractors say it’s a plan that residents simply cannot afford, and that they’re skeptical of how the proposal would be implemented. Supervisor Jim Desmond, a vocal opponent of the policy, called it a plan “to punish all San Diegans by taxing them out of their car.”

“They want to force everyone on to trolleys and buses,” Desmond wrote in a scathing statement Thursday. “This proposal should never see the light of day. San Diegans already pay some of the highest prices to drive in the Country. From the current gas taxes to a vehicle registration tax, San Diegans feel the effects, in their wallets, every day. Adding another tax, to fund public transportation, is a slap in the face.”

Desmond also criticized the county’s execution on a 2004 sales tax that was intended to improve local freeways, too much of which, he says, was used for public transit instead.

Residents who agree with Desmond plan to gather at a La Mesa town hall to protest the tax at 6:30 p.m. Thursday.

SANDAG is expected to meet Friday to discuss the proposal further. It is set to go before the board for a vote in December.

You can review the plans in detail on the agency’s website.