If you have the roads shapefile you can create a buffer zone covering the roadside areas and use the buffer as a mask for the roadside area. process the classification on the fields and complete the post-processing, also you can link the DEM with the RGB image and determine the roadside areas where the grass fields are more flat compared with roadsides, however visual interpretation based on the pattern, shape and area may help where usually the roadsides are not homogeneous in greenness, rectangular along the roads, also the grass fields are connected in harmony square fields with larger area than roadsides.

Improved may mean several practices. Reseeded (every five years or so) , drained and (maximally) fertilized. These three treatments will be often combined or applied in isolation. Moreover, grassland may be a permanent pasture, hay-meadow or a rotation of these uses within the growing season or inter-annually. I would have no problem to classify  improved versus unimproved grassland in an area I know well. Permanent pasture will have a closed perimeter (barbed wire fence, ditch, wooded banks..) and hay-meadows will be open, Fertilized grassland will look greener (nitrogen) in winter, autumn and early spring. Drainage will show in ditches and evenness of grass cover. Undrained grassland will show structural diversity in the grass layer and marshiness after rain. In other words you may want to consider to select your own variables and classification.