Urban streets are laid out as longitudinal stretches of space and their width is split into sidewalks for pedestrians and lanes for vehicle running and parking. The specific width assigned to each function determines its flowing capacity. Considering the respective spatial footprints of modal vehicles, urban planners demonstrate that pedestrians and bikes use public space more efficiently than cars. Yet, traffic engineers and economists keep to modal trip flow rates as the key indicators of the streets' social utility. The article has a twofold objective first of unifying the respective measurements of planners and economists and, second, of investigating the urban conditions under which street capacity is a scarce resource. The first objective is achieved by relating vehicle spatial footprint to vehicular spatial concentration, and vehicle time-area footprint (TAF) to vehicular flow rate. At the link level the demand-supply ratios of TAF and trip flow rate are fairly equivalent. The second objective is addressed at the city level by modeling urban conditions both on the demand and supply sides of people mobility: on the demand side, population density, trip generation rate and average trip length per travel mode and per longitudinal axiality versus, on the supply side, the lateral, "way density" of longitudinal routes and their respective widths and lanes. The model is applied to five cities spanning the range of population density in urban France, considering their respective mobility parameters and street network geometry. It comes out that people density below 4,000 p/km² have some slack street space, hence no scarcity issue, whereas densities above 6,000 p/km² combined to typical street geometry require transit services on the ground and also underground beyond 10,000 p/km².