1. An apparatus for determining the presence or absence of a turnoff in a roadway, the apparatus comprising: a white-line candidate extractor configured to apply image processing to an image of surroundings of a subject vehicle acquired by a vehicle-mounted camera to extract white-line candidates in the roadway; a white-line likelihood calculator configured to calculate, for each of the white-line candidates extracted by the white-line candidate extractor, a likelihood of the white-line candidate; a white-line selector configured to, based on the likelihoods of the white-line candidates calculated by the white-line likelihood calculator, select a white line; and a turnoff determiner configured to calculate a likelihood for one of a plurality of features of the white line selected by the white-line selector, the plurality of features including (1) the lane line is not a compound line, (2) the lane line is a solid line, (3) the lane line is not a tighter curve, (4) a variation in the parallelism between left and right lane lines is greater than a predetermined value, and (5) a variation in the curvature of the lane line is greater than a predetermined value, and the turnoff determiner being configured to determine the presence or absence of a turnoff in the roadway based on the likelihood calculated by the turnoff determiner.