Apparatus for monitoring use of a carriageway, the carriageway having two or more lanes for use by vehicles travelling in a single direction, the apparatus including: a first pair of inductive loops on or in the surface of each lane, the loops in the pair being positioned substantially side-by-side across the lane and the pairs of loops being positioned substantially side-by-side across the carriageway, the width of each loop in a pair being substantially the same such that the two loops together span substantially the full width of the lane, and each loop having a length in the direction of vehicle travel which is substantially shorter than the width of the loop across the lane; a loop controller associated with each loop, each loop controller energising its associated loop and carrying out measurements of the change in inductance of its associated loop as a vehicle passes over that loop; and processing means adapted to receive the measurements from the loop controllers, and adapted to use the measurements to calculate the estimated lateral position of vehicle(s) on the carriageway relative to the centre of a lane which has one of the pairs of inductive loops, the estimated lateral position being derivable from an estimated proportion of a vehicle over one of the loops of the pair of inductive loops, in which the estimated proportion is based on the measured inductance changes using an equation: prop = value − edge value adjacent + value − 2 × edge wherein prop is the estimated proportion of the vehicle over the one of the loops the vehicle passes over; value is the inductance change measured in the one of the loops the vehicle passes over; value adjacent is the inductance change measured in the adjacent loop the vehicle passes over; and edge is a reference value estimating the inductance change caused by a vehicle passing next to, but not over, a loop. a first pair of inductive loops on or in the surface of each lane, the loops in the pair being positioned substantially side-by-side across the lane and the pairs of loops being positioned substantially side-by-side across the carriageway, the width of each loop in a pair being substantially the same such that the two loops together span substantially the full width of the lane, and each loop having a length in the direction of vehicle travel which is substantially shorter than the width of the loop across the lane; a loop controller associated with each loop, each loop controller energising its associated loop and carrying out measurements of the change in inductance of its associated loop as a vehicle passes over that loop; and processing means adapted to receive the measurements from the loop controllers, and adapted to use the measurements to calculate the estimated lateral position of vehicle(s) on the carriageway relative to the centre of a lane which has one of the pairs of inductive loops, the estimated lateral position being derivable from an estimated proportion of a vehicle over one of the loops of the pair of inductive loops, in which the estimated proportion is based on the measured inductance changes using an equation: prop = value − edge value adjacent + value − 2 × edge wherein prop is the estimated proportion of the vehicle over the one of the loops the vehicle passes over; value is the inductance change measured in the one of the loops the vehicle passes over; value adjacent is the inductance change measured in the adjacent loop the vehicle passes over; and edge is a reference value estimating the inductance change caused by a vehicle passing next to, but not over, a loop. prop is the estimated proportion of the vehicle over the one of the loops the vehicle passes over; value is the inductance change measured in the one of the loops the vehicle passes over; value adjacent is the inductance change measured in the adjacent loop the vehicle passes over; and edge is a reference value estimating the inductance change caused by a vehicle passing next to, but not over, a loop.