ANN ARBOR, MI - The state is almost done adding a third lane in both directions to U.S. 23 north of Ann Arbor. It's designed to ease traffic congestion, but it's not a standard highway travel lane.

The so-called "Flex Route" is an extra lane on the inside shoulder of the expressway that is only open during rush hour and times of heavy traffic. Overhead signals tell drivers if the lane is open for use.

A green, downward-pointing arrow above the new lane indicates the flex route lane is open. If the green arrow is present, drivers can use the flex lane.

If a red "X" is showing above the lane instead of a green arrow, the flex route lane is closed and cannot be used.

Lights above the other two lanes can indicate two things. They will either show motorists a suggested speed limit or may show a yellow arrow, indicating a crash or other incident ahead in that lane. The arrows indicate that drivers should merge to a different lane.

Electronic traffic information boards above the right shoulder will tell motorists what traffic conditions are ahead.

MDOT encourages drivers who are unsure what they should do while traveling the flex route to slow down and stay to the right.

The flex route systems are monitored in real time by MDOT workers at the Statewide Transportation Operations Center.

Here's a video that describes MDOT's flex route signaling system.

In the case of a broken-down car or crash, the flex route lane can serve as a refuge if the lane is closed to traffic, said Kari Arend, an MDOT communication representative. However, it's not ideal, Arend said.

MDOT is implementing "accident investigation sites" to assist motorists in that section of U.S. 23, she said. These are areas with a wider right shoulder, Arend said. These areas are the ideal place to park a broken-down vehicle if possible, Arend said.

The flex route signaling system is expected to launch the week of Nov. 13, Arend said, although some sections of the flex route are still under construction.