Baltimore transportation officials shut down one lane of the southbound Jones Falls Expressway Monday night in preparation for an 18-month road widening project - creating a lengthy backup on the highway during yesterday morning's rush hour.

The 1,000-foot-long lane closure between the North Avenue and Maryland Avenue exits will continue for the next 18 months as that section of the JFX is widened to four lanes from the present three.

The city is paying for the $12.5 million project.

But motorists - some of whom reported sitting in traffic for up to 90 minutes - were puzzled to find no work being done yesterday.

Transportation officials said they closed the lane early to condition drivers to the change and because workers had laid down white traffic stripes Monday night to mark the new lane configuration.

Officials planned to put up traffic barriers last night to establish the work zone. They also warned that they may shut down an additional lane during evening hours over the next 18 months.

The North Avenue exit has been closed and will remain so during construction.

Commuters are urged to consider alternate routes. Officials said they expect fewer backups as drivers get used to the lane closure and make accommodations.