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F.Biora
MIZAR Automazione SpA, Via V. Monte 48, Torino, 10126, Italy
S.D.Clark, K.A.Fox, F.O.Montgomery
Institute for Transport Studies, University of Leeds, Leeds, UK, LS2 9JT
Abstract
PRIMAVERA is one of the European Union's Telematics for Transport projects. The main objectives are to develop, test and produce recommendations for the application of integrated traffic control and management measures incorporating queue control, public transport priority and environmental protection.
The work commenced with a review of current techniques in the areas of queue management, bus priority and traffic calming. From review papers for the three areas, 31 techniques were selected, 13 in queue control, 10 in bus priority and 8 in traffic calming. A panel of eleven experts with local knowledge was selected, and this panel ranked each strategy inturn, providing the project with a prioritised list of strategies.
Two corridors were selected for evaluation of the techniques: one in
Torino, Italy and another in Leeds, United Kingdom. Appropriate UTC systems
(SCOOT and SPOT) were modified to incorporate the chosen strategies, which
were then tested using a microsimulation package controlled, in real-time,
by the UTC system. This exercise demonstrated potential synergies and conflicts
between the strategies.
Background
It is unlikely that further extensive motorway construction will take place in urban areas and accordingly, the existing arterial roads will have to be managed with greater efficiency. Such radial routes are often used for motorised trips from outer areas to city centres or even through the city centre and onto another radial road. Often these routes were not designed for such purposes and local planners are restricted by the land use surrounding the road as to how to overcome traffic problems such as congestion, delays to on-street public transport, and the use of residential streets to by-pass bottlenecks on the radial roads.
The European Union is heavily involved in transport projects which are grouped under the heading of Telematics for Transport. One such project is PRIMAVERA, (PRIority MAnagement of Vehicle Efficiency and Road safety on Arterials) whose main objectives are to develop, test and produce recommendations for the application of integrated traffic control and management measures incorporating queue control, public transport priority and traffic calming. The emphasis in PRIMAVERA is not therefore on individual strategies but on the inter-action between strategies. Figure 1 illustrates the area of interest as being the combination of strategies from each area.
Click here for Figure 1 Schematic of the project's area of interest
Field Trial Sites
The first representative sites for the development of these measures are, appropriately, the cities where the research for PRIMAVERA is being conducted, namely Torino, Italy, and Leeds in the United Kingdom. In Torino the site is Corso Grosseto, a major arterial at the city end of which is a grade separated junction with the motorway to the airport. Congestion on the arterial can cause spillback leading to queues of standing traffic on the motorway exit. In Leeds, the site is Dewsbury Road between the A6110 Leeds Outer Ring Road and the M1 Motorway.
Corso Grosseto, is part of the system of arterials connecting the main inter-urban network to the city. The arterial carries approximately 30,000 vehicles per day and near 2,200 per hour in the peak hours. The total study area consists of the trunk of Corso Grosseto leading to Piazza Rebaudengo and the surrounding area. The total site contains seven signal controlled intersections. During the peak hours this arterial is affected by heavy traffic, giving rise to long queues, often blocking the cross flows and impeding the approach of the public transport vehicles to service stops.
Turning to the Leeds site, Dewsbury Road carries about 24,000 vehicles per day, and near 2,000 per hour in the am peak. The character of the road varies from four-lane divided to two or three lanes past local shops. There is a with-flow bus lane over part of the length in the direction outbound from the centre of Leeds. There are six signal-controlled junctions and three pedestrian actuated signalised crossings. In common with most urban radial roads, congestion problems are most severe in the morning-peak period in the inbound direction. At this time, the road carries about 30 buses per hour inbound without any bus priority facilities. Typically, motorists are delayed so they consider using residential streets to try to find alternative routes. This gives rise to environmental problems, such as noise, pollution and danger to pedestrians and other road users. Thus the need for queue management, bus priority and traffic calming is present at this site.
Available Strategies
One of the first tasks in this project was to write three review papers of strategies to overcome the problems likely to be encountered on the test sites. These papers considered queue management (Quinn, 1992), bus priority (Lanteri, 1992), and traffic calming (Harvey, 1992). These were used as a starting point for the development of strategies.
Queue management
In the queue management strategies, the underlying theme is to prevent a queue from spreading along an arterial road and so reducing the capacity of the network as a whole. Traffic control techniques are vital but must be preceded by standard highway engineering methods to maximise the capacity. The following is the list of strategies devised for Dewsbury Road.
1 Restrict turning into the congested arterial by, for example, limiting the amount of green time permitting such a movement;
2 Store queues in the minimum length of road possible, clear of other movements which can bypass the critical junction;
3 Reverse the offset of the signal timing plan from an upstream junction to a downstream critical junction, so that green at the upstream signal is given to the cross street just as the arterial road queue there begins to move;
4 Double the cycle time at the critical junction compared with the cycle time at other junctions;
5 Identify links on which queues can be stored without blocking other junctions or creating further environmental intrusion;
6 Meter the entrance to Dewsbury Road;
7 Use variable message signs to encourage the use of other routes;
8 Use shorter cycle times after demand eases to hasten congestion recovery;
9 Create tidal flow procedure by reversing a lane on Dewsbury Road;
10 Use auto-gating, whereby each link stores vehicles without blocking back.
Bus Priority
11 Priority to buses at signals;
12 Co-ordinate signals for buses;
Traffic Calming using ATT
13 Speed advice using VMS;
14 Speed enforcement using cameras.
15 Link traffic signals to force a lower mean speed between junctions;
It is not feasible to simulate all of these strategies prior to a field trial since the number of potential combinations is very large. Also there is only anecdotal evidence as to the effectiveness of, for instance, VMS for route-guidance (strategy 7) so that simulation of their effect is more a matter of judgement than modelling.
To eliminate some of the strategies from simulation, a panel of eleven experts was assembled, and asked to estimate the effectiveness of each of the strategies. The members of the panel were drawn from the local authority highway and traffic engineers and planners, ITS, MIZAR, bus operators and the West Yorkshire Passenger Transport Executive. Eight impacts were considered:
1 Travel time of people and goods in the corridor.
2 Travel time of commercial vehicles.
3 Fuel consumption, noise and air pollution.
4 Vehicle operating costs.
5 Driver stress.
6 Pedestrian and cyclist stress.
7 Safety.
8 Mode choice.
The experts were asked to consider a list of affected groups. These were: pedestrians, bicyclists, car drivers, bus drivers, taxi drivers, lorry drivers, car passengers, bus passengers, taxi passengers, bus operators, taxi operators, residents and business premises. Each impact for each affected group was then given a score on a five-point scale which simply indicated whether the strategy would result in a significant reduction, a slight reduction, no change, a slight increase or a significant increase. Later, the scores of the experts were added and weighted according to the importance of each of the impacts.
Implemented Strategies
When the priorities identified by the panel of experts and implementation aspects were taken into account the following individual strategies emerged as being worthy of testing. Each strategy is to be tested within one or both of the two traffic responsive UTC systems under consideration, the UK SCOOT system (Hunt, 1981) or the Italian SPOT system (Mauro et al, 1991).
Each strategy is given a key letter for later identification.
Torino - SPOT
E Enhanced SPOT. A number of features such as enhanced synchronisation and a refinement to the systems cost function were added as a result of studies undertaken.
Q Horizontal queue model (Cooperative auto-gating). This strategy performs a link occupancy protection task. This is achieved by the downstream controller sending information about the link occupancy to the upstream controller. The excess link storage capacity is then taken into account in the cost function of the upstream unit. Thus upstream traffic is gated in real time.
B Bus stop protection. Sophisticated bus priority measures already exist on the study corridor. The innovative part of this strategy is the use of the Automatic Vehicle Monitoring system to ensure that bus stops which are usually obstructed by private traffic queues are clear when a bus is predicted to arrive.
C Calming by de-synchronisation. The aim of this strategy is to reduce excessive speeding on a link through the de-synchronisation of signals for a particular direction of traffic.
S Speed advice. This strategy involves advice being given as to which speed drivers should take in order to find a green light at the downstream intersection.
Leeds - SPOT
Q Horizontal Queue Model (Cooperative auto-gating). see above.
A1 Auto-gating 1 - The MX strategy.
A2 Auto-gating 2 - Linear feedback control.
Both these methods involve the calculation of an upper limit to the total length of green time to be allocated to a link, as a function of downstream queues. This value is then used to temporarily overwrite the maximum green time for the phase which is green to the link.
Click here for Figure 2 TIRIS transponders in various housings
source : TIRIS News : Issue no 7, 1992
B Bus priority. Four junctions are equipped with inductive loop reading devices (typically 150m upstream from the stopline) which are able to detect buses fitted with an inexpensive transponder, each of which has its own unique code number. Once such a bus is detected then action can be taken to reduce any delay this bus may experience. Figure 2 shows a photograph of transponders in various housings.
S Speed enforcement. A combination of microwave speed detector, VMS and camera is installed on a section of the field trial arterial. Such an arrangement will help to discourage excessive speeding. The equipment as installed on site is shown in figure 3.
Click here for Figure 3 The VMS installed on Dewsbury road
Leeds - SCOOT
A1 The MX strategy.
A2 Local feedback control.
A3 Linear quadratic co-ordinated feedback control.
A4 Linear quadratic integral LQI control.
All these methods involve the calculation of an upper limit to the total length of green time to be allocated to a link. This value is then used to temporarily overwrite the maximum green time for the phase which is green to the link. Since each method operates in a similar manner only one method was used in combined strategies. Initial simulation results suggested that A1 strategy performed the best and therefore only this strategy, of the four, was used in combination with others.
W Starting and stopping waves. This strategy attempts to adjust the offset between two adjacent junctions to ensure that traffic is released from an upstream junction into the back of a moving queue of vehicles.
B Bus priority. see above.
P Bus progression. This strategy attempts to adjust the offset between two adjacent junctions so as to better represent the increased travel times experienced by buses in comparison to private traffic.
S Speed enforcement. see above.
Simulation Test-bed
In order to select the best combination of strategies to implement in the field a simulation test bed was constructed. This test bed consists of three components:
NEMIS microsimulation package;
An interface program;
A UTC system (SPOT or SCOOT).
The NEMIS microsimulation package (Mauro, 1991) provides traffic flow information to the interface program which then translates it into a form suitable for the UTC system. The UTC system then derives an optimal signal plan based on this information which is communicated back to the interface. The interface then implements the signal timings in the microsimulation package. All this takes place in real-time, which is similar to on-street implementation. Information on the performance of each system can be obtained from indicators collected by the microsimulation package.
Evaluation Methodologies
Three processes are used to assess the detailed and overall impact of each strategy, both in isolation and combination. The first is percentage change in the following impacts:
Mean speed (m/s); Excessive speeding time (s); Duration of spill back (s);
Stops (veh); Delays (s); Travel time (s); Bus travel time (s);
Fuel consumption (l); CO/NOx/HC emissions (g).
The second is a cost benefit analysis using rates given in the DRIVE I EVA manual. This method is simple to apply to those impacts which are easily quantifiable in monetary terms. A disadvantage is the high benefit given to reductions in travel time, far outweighing other impacts.
To overcome the above bias in cost benefit analysis a multi-criteria approach is also used. The percentage changes in impacts are compared to a target percentage and if a measure achieves (or exceeds) the target it is given a full score on that impact. If a measure fails to achieve the target then a reduced percentage of the score is awarded. Thus a degree of flexibility is available in the definition of the target percentage and also the overall score for the impact. Sensitivity analysis is also possible, allowing one to question to what degree the scores would have to change in order to affect the ranking of the alternatives. Three sets of standard weights are available: Official; Environmental and Commercial. In practice the results from the use of commercial and official weights are similar and it is these weights which are used in the following results.
For each strategy the total multi-criteria score can be decomposed into three separate scores which relate to each of the three Telematics for Transport goals of Efficiency; Environment and Safety. These three scores define a point in 3D space which can be plotted, thereby revealing the best strategy.
Simulation Results
Torino
Click here for Figure 4 CBA for Torino strategies
The on-street green wave plan as currently used on-street acts as a comparative base for the Turin results. Figure 4 shows the percentage benefit of each strategy combination over the base case. All strategies show an improvement, the best being E, Q, S, C, Q+S and Q+C.
Click here for Figure 5 3D plot for Torino strategies
Figure 5 shows the 3D multi-criteria plot. The individual strategies (hollow symbols) tend to cluster in the efficient, unsafe and unenvironmentally sensitive region of the plot. The integrated strategies are efficient, safe and environmentally sensitive. A cluster of three strategies appears to score high in terms of all three objectives, S, Q+S and Q+B+S.
Leeds - SPOT
The enhanced SPOT software acts as a comparative base for the Leeds, SPOT results. Here the cost benefit results given in figure 6 show a mixed picture, some strategy combinations show a positive benefit (Q, B, Q+B, Q+S, B+S and Q+B+S) whilst others show a disbenefit (S, A1 and A2).
Click here for Figure 6 CBA for Leeds SPOT strategies
Click here for Figure 7 3D plot of Leeds SPOT strategies
Reference to the multi-criteria plot of figure 7 shows that the integrated strategies perform well on efficiency and environment but poor on safety in comparison with individual strategies.
Leeds - SCOOT
Results from the SCOOT based simulations will be available later on in the year
Conclusion
Thus far the project has demonstrated that it is possible to design integrated strategies in which the individual elements work together in a cooperative manner. The paper has described how the individual strategies were formulated, ranked and selected for testing by simulation. The chosen strategies have been described and results of simulations of the integrated strategies presented.
Field trial of the best integrated strategies resulting from the simulations are being carried out in 1994.
A manual of best practice in the design and implementation of integrated telematics based traffic management schemes for urban arterials will be produced as an output of the project.
References
1. Harvey, T. (1992). A Review of Traffic Calming Techniques. Deliverable No.3, DRIVE II Project V2016: PRIMAVERA.
2. Hunt, P., Robertson, D., Bretherton, R. and Winton, R. (1981). SCOOT - A Traffic Responsive Method of Co-ordinating Signals. Laboratory Report 1014, Transport and Road Research Laboratory, Crowthorne, UK.
3. Lanteri, F. (1992). Features of Public Transport Priority Systems. Deliverable No.2, DRIVE II Project V2016: PRIMAVERA.
4. Mauro V. (1991). Evaluation of dynamic network control : simulation results using NEMIS urban microsimulator. Transportation Research Board Annual Meeting, Washington DC, USA.
5. Mauro, V. and Di Tarranto, C. (1989). SPOT-UTOPIA. In CCCT'89 - AFCET Proceedings, Paris, France.
6. Quinn, D. (1992). A Review of Queue Management Strategies. Deliverable No.1, DRIVE II Project V2016: PRIMAVERA.