Starting on September 26, 2015 a new law will take effect in Washington state that makes it a civil offense to carry marijuana in an opened container inside a motor vehicle.  

This will bring marijuana possession on par with alcohol possession, in that carrying an open container in a motor vehicle will bring a ticket.  

The relevant text of the law states:  

It is a traffic infraction:  for the registered owner of a motor vehicle, or the driver if the registered owner is not then present, or passengers in the vehicle, to keep marijuana in a motor vehicle when the vehicle is upon a highway, (A) in the trunk of the vehicle, (B) in some other area of the vehicle not normally occupied or directly accessible by the driver or passengers if the vehicle does not have a trunk, or (C) in a package, container, or receptacle that has not been opened or the seal broken or contents partially removed. A utility compartment or glove compartment is deemed to be within the area occupied by the driver and passengers.

This means that marijuana users can no longer carry their drug of choice in the passenger compartment, glove box, center console or other interior compartment, unless it is in a sealed container, similar to the law for alcohol.  

Until this time, marijuana could be transported in an open container, as long as a person did not consume it in public.