To　date　immunity　to　disruptions　of　multi-scale　urban　road　networks　(URNs)　has　not　been　effectively　quantified.　This　study　uses　robustness　as　a　meaningful　-　if　partial　-　representation　of　immunity.　We　propose　a　novel　Relative　Area　Index　(RAI)　based　on　traffic　assignment　theory　to　quantitatively　measure　the　robustness　of　URNs　under　global　capacity　degradation　due　to　three　different　types　of　disruptions,　which　takes　into　account　many　realistic　characteristics.　We　also　compare　the　RAI　with　weighted　betweenness　centrality,　a　traditional　topological　metric　of　robustness.　We　employ　six　realistic　URNs　as　case　studies　for　this　comparison.　Our　analysis　shows　that　RAI　is　a　more　effective　measure　of　the　robustness　of　URNs　when　multi-scale　URNs　suffer　from　global　disruptions.　This　improved　effectiveness　is　achieved　because　of　RAI＇s　ability　to　capture　the　effects　of　realistic　network　characteristics　such　as　network　topology,　flow　patterns,　link　capacity,　and　travel　demand.　Also,　the　results　highlight　the　importance　of　central　management　when　URNs　suffer　from　disruptions.　Our　novel　method　may　provide　a　benchmark　tool　for　comparing　robustness　of　multi-scale　URNs,　which　facilitates　the　understanding　and　improvement　of　network　robustness　for　the　planning　and　management　of　URNs.