HONOLULU -- The city had estimated that a line from Kapolei to the University of Hawaii would cost $3 billion. But a new study said it would now cost at least $4.5 billion.

Oahu residents are preparing for a 0.5 percent increase in the excise tax in January to pay for a transit project. That hike will not be enough to cover the $4.5 billion cost of the plan. So Mayor Mufi Hannemann wants a shorter, cheaper option.

A new study found that a fixed-rail transit system would be better than more buses, toll roads or other options on Oahu.

"The study concluded that this would cause the least pollution, consume the least energy, result in the highest increase in transit riders and have the greatest potential to slow the growth rate of traffic congestion," Hannemann said.

Since the cost estimates have sky rocketed, the mayor wants the City Council to choose between two options. One is a 28-mile transit line that would go from Kapolei through downtown and Waikiki and end at the University of Hawaii Manoa. That would cost $4.6 billion.

The second, shorter option would cover only 20 miles, starting midway between Kapolei and Ewa near the site of the future UH West Oahu. It would only go as far as downtown Honolulu and would not extend into Waikiki and UH. The cost would be $3.3 billion.

The mayor supports the shorter option, because the city does not have enough money to build the longer one.

"If you want to do the 28-mile alignment, we don't have those dollars now," Hannemann said.

The city estimates that the shorter line would serve 90,000 people a day, 30,000 fewer riders than the original Kapolei to UH route.

"The concern of mine has been and continues to be cost. And the cost seems to be going up and up and up. When we first were looking at it, it was about $3.3 billion and now it's going up $3.6 billion and perhaps $4.6 billion," Councilman Charles Djou said.

The City Council is expected to choose one of the routes by the end of the year, or the council could decide not to build a transit system.