The proposed tax is structured as a 0.5% general sales tax across those five counties. Notably, the measure does not include a carve-out that previously allowed San Mateo County to retain tax revenues generated within its borders — a feature of an earlier iteration of the proposal that drew negotiation and scrutiny from peninsula officials.

North Bay counties are not part of this measure. A separate transit funding proposal is under development for that region.

The signature submission does not guarantee a place on the ballot. County election officials must verify the signatures before certification. If certified, the measure would go before voters in November and would require a simple majority to pass — a lower threshold than the two-thirds supermajority required for special taxes.

The five-county footprint covers the core of the Bay Area's existing transit network, including BART, Caltrain, AC Transit, and Muni. How revenue would be allocated across agencies and capital versus operations has not been detailed in publicly available campaign materials reviewed for this story.

Sales tax measures place a proportionally larger burden on lower-income households, who spend a greater share of income on taxable goods. That tradeoff is a standard point of contention in regional transit funding debates and is likely to surface during the campaign.

Watch for: county election officials' signature verification results, which will determine whether the measure is formally certified for the November ballot. A campaign finance filing deadline will also reveal who is funding the effort.