unmotorable
/(ˌ)ʌnˈməʊt(ə)rəbl/Now chiefly W. Afr. and Indian English
adjective
Of a road, etc.: not suitable for or usable by a motor vehicle.
Origin
Early 20th century; earliest use found in George Dixon Abraham (1871–1965), photographer and rock-climber. From un- + motorable.