The road becomes Tudworth Road and overlaps the A614; at the Tudworth Roundabout, the A18 continues to the right of the M180 As High Levels Bank, it crosses a flat landscape known as Hatfield Chase and enters North Lincolnshire.

At Crowle, it meets the A161, close to junction 2 of the M180 and crosses Pilfrey Bridge over the Three Rivers.

The A18 meets the B1392 (for Keadby) and passes through Gunness, crossing the B1216 (for Burringham) as Station Road.

[2] The road meets the Briggate Lodge Roundabout on the line of Ermine Street, for junction 4 for the M180 and the trunk-road A15.

At the B1210 (for Brocklesby) junction, the road heads to the right, entering Lincolnshire and the district of West Lindsey, and passes through Great Limber as High Street.

The last section, between the junction with the A46 at Laceby and the junction with the A16 at Ludborough, was designated by the Road Safety Foundation as the most dangerous in the country for car drivers, drawing on collated data on the number of fatal and serious-injury collisions in 2010.

[5] The A18(M) was a 1-mile-long (2 km) motorway that connected the M18, from what is now junction 5 north of Hatfield, South Yorkshire, to the A18.

About 0.5 miles (0.8 km) of the westbound A18(M) carriageway was abandoned when the M180 was built on a new alignment.