As it is predicted that there is a 70% possibility of an earthquake directly hitting Tokyo metropolitan within the next 30 years, it is an urgent issue to ensure the traffic function of emergency transportation roads (hereafter called "ETR"), which is an important infrastructure that supports emergency activities at the time of a large earthquake (e.g., wide-area evacuation, firefighting activities, ambulance services, etc.). In previous paper, we discussed about the transportation function of ETR after a large earthquake by evaluating the accessibility of ETR and the effects of quake-resistant-conversion of roadside buildings. In this paper, we propose the method for extracting vulnerable accessibility roadside areas, which have high possibility to hinder emergency activities after a large earthquake.
First, to extract the vulnerable roadside areas with poor accessibility, we propose novel indices, Link Isolation ratio (hereafter called "LI ratio") and Network Isolation ratio (hereafter called "NI ratio"). Next, we improve a simulation model, which we had previously constructed. This simulation model is consist of the following two sub-models. The one is the road blockage model that describes the collapse of each roadside building based on the construction year and the structure of each building, and describes the road blockage based on the collapse situation of roadside buildings. The other one is the emergency-vehicle-movement model that describes movement of emergency vehicles to activity bases. Then, performing the simulations, we evaluate the accessibility to disaster base hospitals (total 80 bases) using ETR in Tokyo metropolitan. We examine the simulation results with accessibility indices, LI ratio and NI ratio, which are calculated on the assumption that the medical relief activities are conducted at disaster base hospitals in the corresponding secondary healthcare service area.
LI ratio(pmin), the possibility of an emergency vehicle cannot access to any disaster base hospital, indicates higher value than road blockage ratio at 50.0% of road links, and there is a difference of 20.0 points or more between these indices at 15.5% of road links. This is because the isolation status of each road link is result of not only its own blockage status but also surrounding situations. From the spatial distribution of the LI ratio(pmin), we can grasp vulnerable roadside areas with poor accessibility to disaster base hospitals. For instance, it is difﬁcult to secure a detour route when emergency vehicle encounters road blockages in an area with sparse road network. In such areas, isolated links tend to occur over a wide range.
NI ratios(pmin) exceeds 50.0% at 7 disaster base hospitals (8.8% of the total). In the secondary healthcare service areas with those hospitals, there is a high possibility that the medical relief function is biased toward hospitals with high accessibility. Therefore, it is also important to consider the countermeasures to improve the accessibility to hospitals with poor accessibility. Moreover, NI ratios(pmin) indicate high value in secondary healthcare service areas such as Nishi-tama and Kita-tama Hokubu. At the time of a large earthquake, there is a possibility that some roadside areas become unreachable to any hospital and patients in those areas may not be able to receive medical relief activities. To improve NI ratios(pmin) of those areas, there is an urgent need to consider the countermeasures such as quake-resistant-conversion of buildings around disaster base hospitals or road links with high LI ratios.