Writing the programme

The usual way of controlling things for the simulated vehicles is to pass over simulated detectors and the SNMP interface passes the information to the external controller. This decides what action to take either by changing signal times or sending messages by an ITS controller in the model.

The interface for the controller is a SPreadSHeet, it is the run sheet to enter the information. It has been written in Visual Basic macro language because it is accessible and it is ideal for interfacing to the dynamically linked library that implements the SIAS SNMP interface. It is also modifiable without the use of any special software tools and the source code is clearly visible, Figure ‎7.1 shows the SPreadSHeet.

SOFTWARE MODEL

SOFTWARE MODEL

The‎“Run‎simulation”‎button‎runs‎the‎requested‎number‎of‎actions‎of‎the‎specified‎model‎and‎ may or may not log data as required. It consists of several of rows, and the simplification of the SPreadSHeet can be seen in Table ‎7.1. Different versions of this SPreadSHeet will allow different strategy combinations to be tested and the results will be analysed to investigate the effect of strategy choice.

Table ‎7.1 The SPreadSHeet components

SPreadSHeet components Explanation

Paramics Model Location

Model location, where the model is stored in the PC. For example, C:\Paramics evacuation\Controller\Demand strategy Paramics Version S-Paramics ITS system version. In this

study, we use the 2010.1 version

ITS Strategies Sheet

The name of the worksheet with the list of evacuation strategies which are implemented in this study. For example, Demand Strategies

ITS Responses Sheet The name of the worksheet with the list of response profile

Batch or Visual Number of the mode. For example, in this study‎we‎have‎the‎“Visual”‎mode

Number of runs 1

Number of strategies The number of the rows in the strategies sheet

SOFTWARE MODEL

The detail of the parameters for each action will vary with each action type, but will be set on the action row in one or more cells. Once connection is established, the SNMP controller creates the links to the objects in the simulation it intends to manage or to use to collect data, and the simulation is instructed to notify the controller. Some internal controllers will adjust signals and others will do motorway congestion management. All can be broadcasted via either pass the messages to vehicles and there can be a road side sign giving the messages as vehicles pass or it can be a broadcast box giving messages to all vehicles. In other words, active traffic controllers receive data from roadside vehicle detectors and use it for example to adjust the timing of signals in adjacent junctions.

A controller may be in charge of a single junction with one set of detectors or it may coordinate the actions of many junctions with traffic flow data received from multiple sources. Typically this is every second for signal control and every minute for less time critical actions. It passes messages to the control devices in the simulation to modify vehicle behaviour, and then instructs the simulation to restart and run to the next controller time step. The data that could be received from the simulation are:

 From loop detectors in the road, split by lane or aggregated for the carriageway

 Speed

 Flow

 From signals

 Current stage

 Time left in this stage

 From ITS detectors

 Journey time on a selected path

 Queue length

 From buses

SOFTWARE MODEL

 Speed of bus

 Occupancy of bus

 Time at last stop

 From car parks

 Occupancy

 From the vehicle releaser

 The number of vehicles to be released on a specified trip The actions that may be passed to the simulation are:

 To signals

 Move to the next stage now

 Move to a specified stage now

 Change the red or green time for a stage for just the next time it is called

 Change the red or green time for a stage for all subsequent calls

 To flow controlled nodes ( i.e. signalised nodes with no prescribed stage structure)

 Priority of a turn through the node( Major Medium, Minor or Barred)

 Signal state of a turn through the node( Red Amber or Green)

 To the vehicle releaser

 Modify a release rate ( takes effect at the next 5 minutes release rate boundary)

 Release a vehicle on a specified link for a specified trip

 Release a bus on its route

 The actions that may be passed to an ITS device are

 A message to be relayed to vehicles

SOFTWARE MODEL

The ITS device messages are composed of three elements:

 A display message – purely for display and no further details

 Text

 JPG image

 An action message – to interpret the display message and provide an action for the vehicles

 Speed limit

 Lane restrictions

 Route information

o Achieved speed on links o Waypoint delay information o Car park redirection

 Behaviour changes ( awareness and aggression or a headway modifier )

 A response rate – to describe which vehicles take heed of the message, saying which vehicles will pay attention

 Percentage of vehicles

 Vehicles of specified types

 Vehicles with specified destinations

An example of these messages: we can have a picture of a lorry in a red circle which means a lane is closed to some vehicles, specifically Lorries. Part 1 is the image, part 2 is that it is a lane close and part 3 that only vehicles of types 10, 11 and 12 (i.e. the Lorries) are affected. In this application, the ITS devices messages are the key element as they control the actions of vehicles moving in the simulation affecting their wider are responses – i.e. route choice and behaviour.

SOFTWARE MODEL

Finally, if a strategy fails, e.g. the junction is not in the model, or the controller does not exist for a reason. Then the fail routine (fail message) is called. This leaves a message on the connection sheet and clears the connection. In short, it calls the process to stop with no attempt at error recovery. As the error is in the input data there is no error recovery strategy to implement other than respond to the message and correct the input.

For example, as one common error can be seen in describing a behaviour is to mistype the link name,‎i.e.‎“L1:2,‎L2:3,‎L3:4”‎as‎“L1:2:L2:3,L3,4”‎or‎to‎specify‎a‎non-existent link.