MISSOULA, Mont. (AP) - A judge has ruled the Montana Department of Transportation must do a more extensive environmental review before allowing oversized loads of oil refinery equipment to use the state's highways.

State District Judge Ray Dayton on Friday upheld a preliminary injunction he issued last summer that blocked Imperial Oil/Exxon Mobil from transporting more than 200 so-called megaloads through Montana as they move from Idaho to the oil sands of Alberta in Canada.

Dayton went an additional step and ruled the agency must review the oil company's use of alternative routes to haul about 300 smaller, reconfigured modules the company has been shipping since running into problems trying to get legal clearance to move the megaloads.

Dayton ruled the transportation department violated the state Environmental Policy Act when it signed off on allowing the megaloads on U.S. Highway 12 and state Highway 200 in western Montana.

Specifically, Dayton said the agency failed in its review because it never determined whether turnouts built for the Kearl Module Transportation Project to allow the large loads to pull off the highway would remain or be removed once shipments ended. Opponents of the shipments contend the route could become a permanent corridor for oversized loads.

"Without first determining the scope of the project, i.e. whether the turnouts will be permanent or temporary, MDT could not meaningfully assess impacts associated with the KMTP," Dayton wrote in his decision. "MDT therefore violated (the Montana Environmental Policy Act) and its implementing regulations."

Missoula County and three environmental groups sued to stop the transports, arguing the state approved an insufficient environmental assessment.

"I am happy to report that the judge has largely ruled in favor of the plaintiffs, and the matter is being remanded to MDT for further environmental review," Missoula Deputy County Attorney James McCubbin said in a notification to county officials.

After setbacks and court delays caused by residents, conservation groups and local governments opposed to shipping the loads across scenic two-lane highways in Idaho and Montana, the company started chopping up the megaloads into smaller versions that could fit under interstate bridges.

Imperial was cleared last year by Idaho officials to transport the smaller loads through Idaho up U.S. Highway 95 from Lewiston, where the first shipments arrived from the manufacturer in South Korea, to Interstate 90 and east into Montana.

The Montana Transportation Department in November started issuing permits to the oil company to ship about 300 of those smaller versions over interstates 90 and 15 to the northern border and into Canada.

The company by late last year had moved about 35 of those loads. Three of those loads on Wednesday left Lewiston and were expected to be on Interstate 90 in Montana by today.

Imperial Oil officials have said the company applied to transport all of the modules along the interstate route rather than jeopardize plans to have the equipment in place by late 2012, but they still consider the original megaload route along U.S. Highway 12 and Montana Highway 200 a viable way to get the equipment to the Kearl Oil Sands construction site.