CHAPEL HILL - The Orange County commissioners still plan to put a half-cent sales tax on the ballot this fall to help pay for a light rail line between Orange and Durham counties and for better bus service.

Some commissioners question light rail, saying buses could move people more efficiently for less money. But at a Chapel Hill 2020 presentation, planners said the two systems will work together.

"It's not an either or," said David Bonk, transportation planner for Chapel Hill. "It's both."

The plan calls for light rail cars to run from near UNC's Ambulatory Care Center to Duke University and on into Durham. Bus Rapid Transit vehicles in Chapel Hill would run in three corridors: N.C. 86/Martin Luther King Jr. Boulevard, U.S. 15-501/Fordham Boulevard and N.C. 54.

Light rail uses electricity from overhead lines to propel one or more cars at street level, above or below ground.

The advantage of light rail is that you can add cars without having to add operators. Bonk said it's "an open question" whether that savings outweighs the technology's initial higher cost.

The other advantage? Light rail stations attract economic development in ways that buses don't, he said.

Bus Rapid Transit is, however, an integral part of the plan to move people through Chapel Hill and Durham more efficiently, Chapel Hill Transit director Steve Spade said.

Traffic along MLK and N.C. 54 already exceeds 5,000 vehicles per day, almost double the 3,000 level the federal government says justifies higher levels of transit, including light rail.

The advantage of Bus Rapid Transit is that the buses are big - the town's articulated, Slinky-like buses are already a form of BRT - and they run more frequently: every five minutes instead of every 15 minutes during peak hours.

But without the sales tax, the town is at its limit, Bonk said. The town cannot afford to take its articulated buses out of regular traffic lanes, buy software so they can extend green signal lights, or to buy similar buses for the other corridors.

The light rail project, including Bus Rapid Transit, is projected to cost Orange and Durham counties $1.4 billion over 20 to 30 years. Half the money would come from federal and state sources, the rest from the half-cent sales tax and vehicle registration fees.

The Chapel Hill-Durham line is part of a planned regional rail system. Durham County voters approved a half-cent sales tax but have not levied it pending action in Orange and Wake counties.

Craig Benedict, planning and inspections director, showed commissioners a graph of declining sales tax revenue for the county last week. Revenue dropped from $22 million in 2008 to $15 million in 2009 and has declined further since then.

In 2008 the state changed how counties collect tax revenue from a population-based model to a point-of-sale model. The old model gave the county sales tax revenue even if residents shopped in another county. Now, the state distributes revenues based on where a sale took place.

Triangle Transit's financial model assumes revenues will be flat for the next few years, then start to pick up, Benedict said.

If they don't, there may not be enough sales tax to fully fund the project, even if a half-cent tax passes.

"If our revenues stayed flat when there was an assumption of increasing then we (would) have to make adjustments," he said.