ASTRID - Automatic SCOOT Traffic Information Database
SCOOT is a valuable source of traffic information with its network wide detectors allowing a large amount of data to be obtained without large additional costs. The ASTRID database system has been developed to use information from SCOOT to provide a historical background of traffic conditions. The system continuously monitors and stores traffic conditions for later retrieval and analysis. The system can also act as a reference against which to compare current traffic conditions.
The ASTRID data is derived from a special format of SCOOT output which produces compressed data every minute. The data is processed to reduce the size of data files and stored in a form suitable for access by the database programs. This enables information on the current state of the network and the expected state of the network (based on historic data) to be accessible for use by other information or incident detection systems. Data displayed by ASTRID is either collected directly from SCOOT or calculated from stored information. The user can access and display both types of data in the same way. The following are the basic data items collected directly from SCOOT messages.
Basic ASTRID Data
The following data are collected directly from SCOOT messages and stored in the ASTRID database:
The flow in vehicles per hour arriving at a stopline, as modelled by SCOOT.
The total delay in vehicle hours per hour, which is equivalent to the average queue length in vehicles on a link, as modelled by SCOOT.
The percentage of four second intervals during a green period when a detector is occupied by traffic. This value is independent of the SCOOT model.
- Detector Flow
The flow recorded as vehicles cross a SCOOT detector. The parameter is not suitable for use in the SCOOT model, but it provides an accurate flow count on single lane links and other links that have a detector per lane.
- Detector Occupancy
A value for the occupancy of a detector calculated by taking the total number of quarter-seconds for which the detector is occupied as a percentage of the whole period.
Derived ASTRID Data
The following data are calculated from information provided by SCOOT, which is stored in the ASTRID database:
- Vehicle Delay
Vehicle delay (or delay per vehicle) is calculated by dividing delay by flow, giving a value in seconds. It is an estimate of delay encountered by vehicles on a link and is unaffected by the number of LPUs per vehicle on the link.
- Journey Time
Journey time is obtained by adding vehicle delay to the 'cruise' time of vehicles between a SCOOT detector and stopline. It is an estimate of the time a typical vehicle takes to travel along a link at an average speed.
Speed is derived from link length, cruise time and vehicle delay.
- Congestion Index
A congestion index is derived from vehicle delay and cruise time.
- Vehicle Occupancy
Vehicle occupancy shows the average occupancy of a vehicle over a detector and is calculated by dividing the detector occupancy by the detector flow.
- LPU Factor
SCOOT processes detector data into its internal units, Link Profile Units (LPU). An LPU is a hybrid of flow and occupancy. The flow in LPU can be related to vehicle flow by the use of an appropriate LPU conversion factor.
ASTRID Data Levels
Data is available at the following levels:
Individual detectors cover a maximum of two lanes in one direction.
A SCOOT link will generally be between two signalised junctions.
Includes all links at an individual node or junction.
The nodes in a SCOOT network are divided into regions.
Any group of links may be defined as a route, but routes required must be specified at the data collection stage.
Provides a summary for the whole SCOOT controlled area.
ASTRID GraphsASTRID provides the following types of graph, which may also be output as tabulated data:
Shows the profile of a data item for individual dates.
Shows the profile for a data item for a day of the week, together with the profiles of maximum and minimum values recorded on the same day of the week.
- Standard Deviation
Shows the profile together with graphs of two standard deviations above and below the profile.