This paper deals with two problems in relation to the accident treatment of urban street networks, ie accident prevention (AP) and accident mitigation (MP). These two problems are defined based on the concepts of suitable remaining trip-hours, link importance, and klink connectedness of node-destinations in the network. The objective of problem (AP) is to upgrade the important links so as to maximize a measure of the performance of the network. This is a before-accident treatment of the network. The objective of problem (MP) is to mitigate the accident effects in the network so as to maximize a measure of network connectedness. This is an after-accident treatment of the network, which is done by equipping