Keep Left, a message on a sign indicating that travelers should keep to the left, especially drivers or walkers

left-hand side

needed, necessary

need, necessity

traffic, passage

pass, go through

a one-way street / (by extension) a one-way transmission especially one without a reverse transmission in exchange

road, street, a way for local traffic

necessity (quality or state of being necessary, unavoidable, or absolutely requisite)

unnecessary