Manteca motorists on North Main Street expecting to drive onto northbound Highway 99 will be directed by a flagman to a detour taking them to Lathrop Road and its northbound on ramp from the Frontage Road.

The North Main Street onramp that has bridged the freeway’s four north and south lanes for years is being closed due to the ongoing construction that is part of the $150 million widening project to three lanes in each direction for the freeway.

The closure and detour took effect Monday morning at 7 a.m.