Network Analyst extension was used in conjunction with the ArcView 3. 2 desktop GIS package to carry the analysis for this system. Network Analyst has the ability to calculate the best route (quickest) to get from one location to another on road networks. The start and destination points can be specified by pointing on desired location on the screen or by entering addresses. In this system, calculating the quickest route or least cost impedance on roads networks is based on different variables associated to the roads from any incident location to the hospital location on road networks.
The results were mapped using ArcGIS 9. 1 by exporting the analysed results as a shapefile from ArcView 3. 2. In the evaluation procedure, NAVTEQ Navigation System which is embedded within Dell Axim X51v PDA was used. Leicestershire road networks was provided by Ordinance Survey UK using OS masterMAP. Road types can be classified as follows: motorway, A road, B road, minor road, local street, private road – publicly accessible, private road – restricted access, alley and pedestrians’ street. While the nature of road can be classified as follows: single carriageway, dual carriageway, slip road, roundabout and traffic island.
A sample of 3*3 KM of this network was clipped for faster analysis and easier for the field evaluation procedure. For the purpose of this research, the length of each segment was converted from metre to km by dividing them by 1000. Some other data layers were added for the purpose of analysis and evaluation of the system such as a hospital and obstacles. A general overview of the study area is shown is shown below. Speed limits of the road network in km/h which was converted from mile/h by multiplying them by 1. 6 (1mile ? 1.
6km) were assigned as a numeric field to the roads according to their nature rather than the type of roads for more accuracy. For example, A road type might have different nature of roads in which each has a different speed limit. This assignment was done by joining a . dpf Excel file that consists of nature and speeds limits fields to road networks using the nature field (Roads) as seen in table 3. Since the accurate driving times on the road network are normally unidentified, they have to be estimated using road lengths and driving speed limits along each road.
The driving time along each road can be then calculated using the formula (Time = road length / Speed) as seen in table 4. The travel time along each segment was first calculated in hours then converted into minutes. Note: For the analysis purpose of this research travel time field’s name was changed into MINUTES in order to be identified as a cost field by Network Analyst, this will be discussed in detail. Negative values were assigned to the travel time cost field to identify a line feature as being closed to travel. Pedestrianised streets and private road – restricted access were assigned to negative values as seen in table 5.
As a result of this the Network Analyst will avoid passing these roads when calculating the fastest path. One-way streets are represented in the road network by adding a string field named as “ONEWAY”. Three directions are assigned to this field, “FT”, “TF”, “N” or an empty value. “FT” means that travel is allowed from the start to the end of a road segment, “TF” means that travel is allowed from the end to the start of a road segment, while “N” means that there is no access or restricted. Finally empty value means both ways are allowed as seen in table 6.
The travel time costs’ fields were given different names in order to be identified by Network Analyst cost field property. The different names were given according to different scenarios which will be discussed later. Network Analyst only uses a set of predefined cost fields names such as minutes, kilometers, meters an so on (ESRI, 1998). In the cost field, minutes represents the travel time along each road while the other travel times represents the weighted travel times that are calculated by multiplying minutes with the weights as will be discussed in scenarios as seen in figure 2 .
There are two ways to add hospital (start) and incident (destination) locations in Network Analyst in order to find the routing. They are either added from already saved layers or by making use of Network Analyst’s add location tool. In this research the hospital location was already stored in a layer while incident was added using the add location tool as seen below. This part aims to integrate ambulance drivers’ knowledge into the system as a set of rules acquired from previous literatures.
Integrating expert knowledge to the system will make it possible to find a more accurate travel time along each road that considers different criteria acquired from ambulance drivers knowledge which affect the travel time. The system considers general rules such as the day and time. For example, certain roads are congested at specific times and 200 metres around specific schools will be congested during the hours of entry and exit. Another example, in 7th of March there will be a football match in Leicester City Football Club, so 500 metres zone around the stadium will be congested before and after the match.