What a difference an hour can make.

The state's extended new Zipper Lane — providing a continuous 15-mile freeway express route for O'ahu's car-pooling commuters — can be clogged, or relatively empty, from one hour to the next, according to preliminary figures.

At times, there are only a handful of vehicles in the car-pool lane, giving users a clear advantage over single-occupant vehicles in adjacent lanes; at other times there are so many vehicles that traffic slows noticeably, prompting complaints from some motorists that they are worse off than before.

State officials say they are taking a wait-and-see attitude before making changes.

"We want to make sure people don't have to slow down or come to a complete stop. We'll continue to monitor the Zipper Lane for a few months to see what changes need to be made," state Transportation Director Rod Haraga said.

One change might be to again increase the number of people required to be in each car-pool vehicle from two to three during peak hours, he said.

Nearly 4,000 people a day are using the extended Zipper Lane on H-1 Freeway, up slightly from use of the shorter lane a year earlier, according to Department of Transportation figures compiled during the first week of operation beginning Aug. 22.

On the busiest day that week, 4,089 cars — or more than 13 percent of all cars on the freeway — used the Zipper between 4 a.m. and 9 a.m., an increase of 20.9 percent from the previous year.

Most of the time the Zipper traffic volume, measured near the Halawa Interchange, remains well below 1,400 cars per hour, the point at which traffic begins to slow to below 55 mph. At the same time, the four other freeway lanes were often well above the 1,400-vehicle threshold.

In the busiest period between 6 a.m. and 7 a.m., however, Zipper use rose as high as 1,736 in the first week, slowing traffic to the same crawl as in the other lanes, officials said.

Some drivers suggest that may be the result of a rule change in August that allows drivers with two or more people in a vehicle — instead of three — to use the car-pool lanes.

Others say the publicity surrounding the new Zipper Lane and its ease of use may be pushing it to near capacity in the busiest times.

"I have conducted my own little experiment and have found that in taking the Zipper Lane this year, my two sons and I not only need to leave 25 minutes earlier than last year, but over the last two weeks drive time has been cut an average of 17 minutes by staying out of the Zipper," said 'Ewa Beach resident Kurt Schweitzer, who suggested that the state revert to the three-in-a-car requirement.

Others, though, report a significant improvement of up to 10 minutes in their commuting time by using the Zipper Lane.

"The extension of the zipper lane and the change to allow two drivers to use the zipper lane has made a world of difference for traffic in all other lanes going eastbound. Last school year, it was not unusual for it to always be backed up past the Kunia on-ramp. This year, even with our share of stalled cars, the most it has been backed up is to the Waikele Shopping Center," one Kapolei commuter said in an e-mail to DOT.

Haraga said the different experiences are likely the result of where and when the drivers are entering the Zipper Lane.

The biggest Zipper slowdowns appear to occur at points where vehicles can enter the special lane, which is open from 4 a.m. to 9 a.m.

"There are some pinch points in the merge areas that can cause a problem. But we think it may get better as people become acquainted with the driving patterns. And others may make adjustments when they see there are fewer problems if they start a little earlier or later," Haraga said.

Reach Mike Leidemann at mleidemann@honoluluadvertiser.com.