Significant progress has been achieved recently about the construction of FUA defined from commuting flows. However, several questions remain about the comparability of such urban objects, due to the strong heterogeneity of LAU2 size and of settlement contexts. This report aims at assessing the feasibility of another method based on accessibility isochrones (including speed parameters) and to question the complementarity between both methods used for defining FUA. For a set of about 10 large cities, urban isochrones perimeters are constructed from different congestion models focusing on road trips. They raise new questions when compared to FUA constructed from commuting flows.