SCOOT MMX (SCOOT Multi Modal 2010)SCOOT MMX provides additional facilities to prioritise pedestrians at junctions, a significant update and enhancement of emissions estimates as well as features to improve operation during low flow periods. It has the following main new features incorporated:
- Pedestrians: variable invitation to cross
- Cycle time sub-region independence
- Reduced cycle time at quiet times
- Updated and enhanced emissions estimates
PEDESTRIANS: VARIABLE INVITATION TO CROSS
In the last release SCOOT MC3 Service Pack 1 a facility was included that allowed users to give greater priority to pedestrians at signalised pedestrian crossings (mid-block crossings). For SCOOT MMX, a new facility targeted primarily at traffic junctions with pedestrian crossing facilities has been developed. In particular it enables priority to be given to pedestrians at sites where the pedestrian demand is high. Where there are large numbers of pedestrians waiting to cross the green man invitation period (and hence the overall time available to pedestrians to cross) can now be extended.
Care was required to ensure that the longer pedestrian period does not result in longer cycle times due to the reduced vehicle green times. Longer cycle times will result in longer waiting times for pedestrians and this would defeat the objective. To ensure that the cycle time is not affected, an increase in the pedestrian green time is coupled with an increase in the 'isat' value (preset target degree of saturation parameter which is normally 90%). This will have the effect of potentially increasing the delay to vehicles on the busiest links of the junction.
Users are able to set up to 4 priority levels. For each pedestrian priority level users can specify the increase in pedestrian green time (in seconds) and the 'isat' value to which the vehicle degree of saturation at the junction is allowed to rise.
CYCLE TIME SUB-REGION INDEPENDENCE
Under normal operation, SCOOT maintains coordination between nodes in a region. In order to do this, the cycle time of all the nodes must be the same, or strictly half in the case of double cycling. At quiet times, however, the benefits of coordination can be reduced. If one node in the region is experiencing heavy traffic it will increase the cycle time of the whole region, increasing the delay at otherwise lightly loaded junctions. These lightly loaded junctions would benefit from a lower cycle time, and there would be an overall region benefit if the cycle time of these junctions was lower than the other junctions, as long as the disbenefits due to lack of coordination do not outweigh this benefit.
This new development 'cycle time sub-region independence' makes it possible for nodes to operate at a cycle time that is independent of the regional cycle time. Whilst coordination will be lost, operating at lower cycle time will reduce waiting times at red signals. The mechanism is set up within SCOOT to enable the independence, and triggers and thresholds established such that the process can also occur automatically if desired.
Often a particular set of nodes work best if they have the same cycle time irrespective of other conditions. Therefore the concept of a sub-region has been established. A sub-region is a set of nodes that will always be considered for cycle time independence together. None of the nodes within the sub-region can operate at a different cycle time from the others. If the sub-region is selected to operate independently from the region, the internal links within the sub-region will still be coordinated.
REDUCED CYCLE TIME AT QUIET TIMES
At quiet times it is usually desirable to keep the cycle time as low as possible. In some regions, however, the cycle time is kept higher than necessary because of the minimum cycle time constraint at one of the junctions in the region.
The minimum cycle time of junctions is calculated such as to allow all stages to run for their minimum time. This makes sense where all stages are permanent or where demand dependent stages are frequently called. However, at quiet times when demand dependent stages e.g. pedestrian stages are called infrequently this can result in the cycle time being higher than desired as the minimum cycle time must cater for the pedestrian stage being called.
In SCOOT MMX, logic has been developed to overcome this problem. The concept of a "ghost" stage has been introduced which is defined as a demand dependent stage that has effectively been omitted from contributing to the node minimum cycle time. The user now has the option of allowing demand depend dependent stages to be treated as ghost stages at chosen times of day. The effect of this is that at quiet times the minimum cycle time of affected nodes will be reduced. In many regions this should result in a lower region cycle time.
The development allows some nodes to operate at a cycle time that is only long enough to accommodate their non-demand dependent stages (plus 4 seconds to enable the offset optimiser to make an advance change). Whilst over the region this will be greatly beneficial, when a demand dependent stage is run at such a node, the cycle time at the node will exceed the SCOOT region cycle time and it will become out of step with the other nodes in the region. In practice the running of the demand dependent stage will have created a bad offset at the node.
Running a bad offset too often will cause a significant increase in delay. To prevent this happening, the frequency of occurrence is assessed over a period of 15 cycles. The demand dependent stage is included in a node's minimum cycle time if the stage has occurred 4 or more times in the last 15 cycles. Similarly, it is removed from the calculation of minimum cycle time, if removal is allowed at this time, when it has not occurred more than once in the last 15 cycles.
UPDATED EMISSION ESTIMATES
Emissions from road traffic are an important source of air pollution in urban areas. Estimates of emissions from vehicles in a SCOOT controlled area were included in Version 4 based on research undertaken in 1995 to 1996. Considerable changes have taken place since 1995:
- A better understanding of the emissions performance of different classes of vehicles as a function of driving conditions through the major EU research project ARTEMIS and work funded by DfT.
- Changes in the performance of vehicles as new regulations EUII, EUIII, EUIV have been introduced.
- Changes in the mix of vehicles on the road, an increasing proportion of cleaner vehicles as new vehicles meeting stricter emissions standards are introduced and old dirty vehicles are retired. Recently the large increase in the popularity of diesel powered cars has changed the emissions pattern.
The emissions estimates have now been updated to provide more realistic estimates of:
- Carbon monoxide
- Oxides of nitrogen
- Particulate matter
- Carbon dioxide
In addition to these pollutants that were originally estimated, two new estimates are provided:
- Total carbon
- Fuel consumption
Fuel consumption is estimated from the total carbon emission using the carbon balance approach.
SCOOT MMX Service Pack 1 provides additional features to SCOOT.