3. (a) Explain the potential advantages of coordinating adjacent traffic signals on a highway network and briefly describe the main features of the methods commonly used for achieving such coordination in the UK (i.e. TRANSYT and SCOOT).
Traffic signal coordination is the timing of traffic signals so that a majority of traffic on the heavier-used road can pass through the intersection on a green light. The concept is that when the first cars in a large pack of traffic approaches a light, the signal should turn green right before braking would have to occur for a red light. If coordination works properly, all the lights on a street should turn green right before major groups of cars approach. This ensures traffic on primary roads don't have to stop every two or three lights waiting for side street traffic. Since traffic on the primary roads do not have to slow, congestion does not occur. Congestion occurs when a car in front brakes causing a chain reaction where everyone behind brakes as well.
This is a great system--in theory. In reality, things are more complex. When an automobile is on a primary road sitting at a red light, traffic from the other directions are filling the road ahead of the driver. This pretty much negates many advantages to traffic signal coordination. As long as build-ups occur further up the road, the signals will never be perfectly coordinated. Therefore, traffic stuck in the back of a pack will get caught at a red light eventually.
Coordination of traffic signals can be used to implement a big range of strategies. Reducing delays and number of stops can be implemented with the creation of green wave which is the process in which a platoon moves through a sequence of traffic signals without stopping. This process reduces significantly the journey times and also helps in reducing the noise and air pollution. Poorly timed traffic signals lead to start-and-stop activity and low travel speeds, which greatly impacts carbon emissions. A program should be developed to make sure the coordinated signal timings are maintained and updated at least every 5 years, and a maintenance program should be developed to quickly repair vehicle detection devices and maintain efficient traffic signal operations. Implementing new signal timing could reduce fuel consumption up to 10% and reduce harmful emissions up to 22%. (Peters, McCourt, & Hurtado, 2009) With the coordinating of traffic signals people may be encouraging in using the optimized route and this will result in attracting cars away from residential roads. Another advantage of traffic signals coordinating is the reducing of queue lengths which ensures queues do not extent back to preceding intersections. Improved speed and reliability of buses can also be achieved by giving priority to buses using selective vehicle detection and emergency vehicles can also be prioritized using this system. (Mountain, 2010)
TRANSYTis an international product, used by hundreds of consultancies and local authorities around the world. It is an off-line computer program for determining and studying optimum fixed-time co-ordinated traffic signal timings in any network of roadsfor which the average traffic flows areknown. Its use around the world reflects the product's international appropriateness and flexibility as a design and network optimization too.
TRANSYT models time-varying traffic conditions, producing signal timings which are optimized for the complete time period and capacity, queue and delay results for each condition and for the overall situation. The model also provides for give-way priority control, including the modeling of opposed right-turn traffic within signaled junctions. The program can be used to produce timings which give priority to buses, trams or emergency vehicles, without the need to detect individual special vehicles within mixed traffic streams. Fully-signalized or partially-signalized roundabouts can be modeled and their delay minimized by calculating timings which reduce blocking-back by keeping the circulating carriageway free flowing. Where blocking does occur TRANSYT's cell-transmission model can be used to model its effects. Weighting factors and queue length penalties can be applied to individual links in order to encourage the optimizer to produce timings that avoid queues beyond a certain length or to reduce delays on specific links. TRANSYTincludes the accurate modeling of flared (short lane) approaches. This is particularly important in modeling signalized roundabout approaches where flares are common. Individual signalizedjunctions (with and without internal stop-lines) can be easily modeled in TRANSYT. Both isolated and 'within junction' signaled pedestrian crossings can be represented within the TRANSYT network model andthe provision of green timesforpedestriansreported. It is suitable for both drive-on-the-left and drive-on-the-right operation. (TRL Software)
SCOOT coordinates the operation of all the traffic signals in an area to give good progression to vehicles through the network. Whilst coordinating all the signals, it responds intelligently and continuously as traffic flow changes and fluctuates throughout the day. It removes the dependence of less sophisticated systems on signal plans, which have to be expensively updated.
Information on the physical layout of the road network and how the traffic signals control the individual traffic streams are stored in the SCOOT database. Any adaptive traffic control system relies upon good detection of the current conditions in real-time to allow a quick and effective response to any changes in the current traffic situation. SCOOT detects vehicles at the start of each approach to every controlled intersection. It models the progression of the traffic from the detector through the stop line, taking due account of the state of the signals and any consequent queues. The information from the model is used to optimize the signals to minimize the network delay.
SCOOT was originally designed to control dense urban networks, such as large towns and cities. It is also successful in small networks, especially for areas where traffic patterns are unpredictable. With over 200 systems worldwide SCOOT is working effectively in a wide range of conditions in places as diverse as big congested cities: Beijing, Bangkok and London, to small towns or networks such as: Heathrow airport and systems localised round individual junctions of the M25. (SCOOT SOFTWARE)