Bids for a roundabout on Edmonds and Fifth Street are $85,724 higher than estimated - and more than a stoplight would cost at the intersection.
The roundabout, planned to replace the temporary circular intersection in place now, had been estimated at $120,000 - $30,000 less than a stoplight.
However, the low bid received this week was $205,724 from Anchor Concrete of Sparks.
Officials will go ahead with construction, though, because a roundabout would cost less in the long run and it would take too long to switch plans to a stop light, they said.
The price tag on the project rose partly because Carson City Regional Transportation Commission engineers underestimated the cost of items such as concrete.
RTC Engineer Harvey Brotzman said he didn't take into account that much of the construction requiring concrete will have to be done by hand rather than by machine. The difference, and the cost of using colored concrete to make it "prettier," added $73,625 to the cost of the project.
Water lines and materials for future landscaping of the roundabout, as well as the cost of moving electrical service, added $4,800 .
Brotzman said because the cost was higher, the city considered alternatives to the roundabout such as reverting to a traffic signal with a cost of $180,000. But it would be another eight months before a traffic signal could be installed. Taking some costs like the colored concrete out would force the city to rebid the project, creating another delay, Brotzman said.
RTC Chairman Jon Plank pointed out that stop lights have an ongoing maintenance cost of $4,000 to $6,000 a year, and over time, the roundabout will cost less in maintenance.
Construction on the roundabout is expected to be completed by July 31, Brotzman said.
The existing roundabout is not a true circle. It has a 45-foot diameter across the narrow portion, and a 65-foot diameter on the longer side.
North/south traffic in the current roundabout has a fairly straight shot through, which has been one of the major complaints.
The new design is a circle with a 78-foot diameter. Instead of going straight through, north/south traffic will have to go around the circle. The design creates a larger center which throws the north and southbound lanes out of alignment.
Islands separating lanes of traffic coming in and out of the roundabout onto Edmonds and Fifth will also be redesigned to accommodate the wider design.
Some Carson residents have been outspoken critics of the roundabout, saying it is dangerous. Others applaud the traffic device for keeping traffic moving.
About 10,000 cars travel north to south and 6,000 cars travel east to west through the intersection daily.
The Nevada Department of Transportation built the existing roundabout as an experiment in April 1999, and the city decided to make it permanent in September 1999.