In 1917, Dr McCarroll was driving her Model T down a stretch of highway when she was almost run off the road by a truck. The experience spurred her to invent the white line down the middle of the road to separate lanes of highway traffic.

Unfortunately, at the time, the very good idea fell on deaf ears at the local chamber of commerce. So McCarroll went out paintbrush in hand and personally painted a white stripe down the middle of the road herself.

Today McCarroll is credited by the California Department of Transportation with the idea of delineating highways with a painted line to separate lanes of highway traffic.

However, there are contradictory historic facts about who was the first to have the highway lines idea. Probably because news traveled so slow, and painted lines to divide the road lanes wasn't considered news

Edward N. Hines from the Wayne County, Michigan road commission, claims to have come up with painted road lines six years earlier. 

In addition, accounts say that two other men painted highway lines in 1917 in other parts of the country: 

A white line along “Dead Man’s Curve” in Marquette County, Michigan by Kenneth Ingalls Sawyer, https://justacarguy.blogspot.com/2014/10/dead-mans-curve-update-vintage-post.html

and a yellow line down the center of the Columbia River Highway in Oregon by Deputy Sheriff Peter Rexford.

https://www.atlasobscura.com/places/doctor-june-mccarroll-monument