Here is an explanation of what all those road lines mean, taken from the California Driver Handbook. You may want to pass this along to your drivers as a friendly reminder. 

Solid yellow lines mark the center of a road used for two-way traffic. 

Broken yellow lines mean you may pass if the broken line is next to your driving lane. 

Two solid yellow lines mean no passing. Never drive to the left of these lines unless you are:

• Turning left at an intersection.

• Turning into or out of a private road or driveway.

• In a carpool lane that has a des­ignated entrance on the left.

• Instructed to drive on the other side of the road because your side is closed or blocked.  

Two sets of solid double yellow lines spaced two or more feet apart are considered a barrier. Do not drive on or over this barrier or make a left turn or a U-turn across it except at designated openings. 

Solid white lines mark traffic lanes going in the same direction, such as one-way streets. 

Broken white lines separate traffic lanes on roads with two or more lanes in the same direction.