We've reached the halfway point on the traffic-snarling road construction projects on the Bloomington Ferry Bridge (Hwy. 169) and on I-694 through the northern suburbs. Hallelujah!

But for many impatient motorists, the end can't come soon enough. They wonder why crews don't work 24 hours to get the job done faster.

"There should be fewer projects, worked on 24 hours a day, 7 days a week until they are done, then move on to the next one," said one reader who commented on my column of two weeks ago in which I explained the reasons for the lengths of construction zones. "Anything less shows a complete disregard for the taxpayers."

It's true that this is an abnormally intense construction season, with 87 projects taking out lanes or shutting down entire roads in the metro area this summer. But even if MnDOT had only 50, working 'round the clock won't make concrete cure any faster.

Night work also is much more difficult and dangerous for both workers and motorists. But the bottom line is that a 24-hour operation adds considerable cost to a project, said Kent Barnard, a MnDOT spokesman. Additional expenses include overtime and higher night pay for crews, higher electricity costs for lighting and setting up additional traffic control.

Some parts of a job can't be done at night. Most plants that supply asphalt or concrete are closed at night. The few that stay open charge a premium, he said.

"If a contractor bids $21 million for the project, they have a deadline and time frame to close lanes and ramps," Barnard said.

"They bid the project and any overtime they pay costs them. They won't throw lots of overtime or incur extra expenses unless there is incentive to do so."

Backups like we have seen on Hwy. 169 through Bloomington and Eden Prairie are to end on Aug. 23. Commuters on I-694 will have to put up with road and bridge work until late October or early November.

Passing through those work zones or traveling on roads that have absorbed extra traffic as drivers look to avoid them brings inconveniences twice a day.

What we often forget is that construction projects affect more people than motorists.

For those who live near a construction zone, night hours are the only respite they get from the ongoing grind. That is another reason darkness brings a work stoppage.

"You as a driver may want to see it [all-night construction], but if you lived next to a freeway would you want vehicles backing up all night and power plants with lights shining in your back yard?" Barnard said.

"We get complaints from people where construction is going on all night. They say it's bad enough during the day, but at night there's the noise, light pollution and vibrations. There are lots of side effects to construction and we have to be aware of that, too."

MnDOT will do overnight work in cases where benefits to the public outweigh the additional costs. That happened when the agency rebuilt the I-35W bridge in Minneapolis after the collapse in August 2007.

"We had to weigh the alternatives for 150,000 vehicles, the cost of not having that route vs. putting traffic on other streets," Barnard said.

"Sometimes we can't win for losing."

Follow news about traffic and commuting at The Drive on startribune.com. Got traffic or transportation questions, or story ideas? E-mail drive@startribune.com, tweet @stribdrive or call Tim Harlow at 612-673-7768.