INDOT officials explain signal timing at SR 135 and Smith Valley Road

Drivers on State Road 135 in Greenwood may have noticed a change in the timing of signals at Smith Valley Road.

The change is called a lead/lag configuration, which means traffic that arrives first at the intersection gets to turn first. The signal timing isn’t new in Johnson County, it is also in place on U.S. 31 in Greenwood and at Interstate 65 interchanges at Whiteland Road and Main Street, Indiana Department of Transportation spokesperson Natalie Garrett said in an email.

The goal is to reduce queuing at high-traffic intersections. So far, the change has improved traffic flow, but it has left some drivers confused, Garrett said.

“The purpose of adjusting when a left-turn phase occurs in a signal sequence is to increase the given time for that specific movement, therefore allowing more vehicles to pass through the intersection and reducing delays and congestion at the intersection,” Garrett said. “Overall, the lead/lag configuration has improved traffic flow along State Road 135 significantly. Queueing has also been reduced on Smith Valley Road. However, the change has caused some drivers, specifically those turning left, not to proceed through the intersection. Drivers should be paying attention to and following the designated signal for their specific movement.”

The lead/lag configuration only applies to north-south traffic at State Road 135 and Smith Valley Road, not east-west traffic.

Garrett explained how the intersection works:

Instead of the typical simultaneous left turns that occur at most four-way intersections with stoplights, traffic that arrives to the intersection first gets a left turn green light, while thru traffic also has a green light. Traffic in the opposite direction waits until their light for thru traffic turns green following the completion of the left turn light in the other direction.

When traffic is flowing in both directions, a yellow flashing turn light means drivers can turn with caution, provided there a wide enough gap in oncoming traffic to make the turn. The signal sequence ends with a solid green left turn arrow for the lagging direction, as the first direction of traffic stops for a red light.

For drivers making a left turn: a red arrow means stop, a green arrow means go, but a solid yellow means prepare to stop and a flashing yellow tells drivers to proceed with caution.