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Hint
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Answer
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Alternative Strategies for network junction control#
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1. Fixed time control during time periods when overloaded conditions are expected + Vehicle Actuated at other times.
2. Measure traffic characteristics across the network and compute optimal combination of cycle time, Phase/stage split and offsets to either minimise network delay or maximise network capacity, + allow local optimisation of intersection control (early termination, skip phases).
3. As above but use a forecasting model to anticipate traffic and hence optimum settings\ |
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Car following#
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Driver behaviour combined with vehicle characteristic leads to variations in speed, deceleration and acceleration
rection times can lead to excessive braking, forming shockwaves and traffic growth\ |
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Clearance time#
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Ensures that two incompatible stream that have green successively dont collide
creates time gap between end of green phase and start of green for the next phase\ |
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Collision classification#
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Fatal
Serious injury
slight injury
damage only\ |
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Conflict#
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A conflict between two or more phases exists if these cannot receive green at the same time\
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Congestion Management#
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Lower vehicle speeds
reduce lane changing
reduce variability in headway
more stable 'laminar' flow\ |
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Congestion management algorithm#
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Set signals to show variable speed limits based on traffic congestion
Measure flow and speed at HIOCC detection sites
Change accordingly\ |
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Coordination of junctions#
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Green wave if green times are synchronised for successive junctions
most heavily used junction will have the longest cycle time so this is used as teh common cycle time for the network\ |
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Crossing design#
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Account for 'desire paths' where ped would walk if unrestrained
Have a clearance period after green\ |
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Cyclist measures#
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Cycle lanes with separate barriers
Signs and routes
Toucan and Soarrow crossings\ |
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Dynamic control in networks#
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Optimum for network is not the same as optimum for individual junction
Actual pattern varies from survey
need for regular survey and update to keep timings relevant
traffic leaving upstream junction may get red at next junction so network is inefficient\ |
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Dynamic Hard shoulder operations#
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before opening hard shoulder check with cctv to make sure it is all clear and signal accordingly image\
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Dynamic Network control systems#
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SCOOT - Split Cycle Offset Optimisation Timing
SCATS - Sydney Coordinated Adaptive Traffic System\ |
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Effective green time#
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Equivalent time of discharge of a queue with a constant departure rate (saturation flow)\
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Factors affecting saturation flow#
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Direction of traffic
external factors - lane width, turning radius, slop\ |
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Fatal Collision#
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Individual dead afet certain number of days after collision\
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Fixed time signal variables#
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Degree of saturation - measure of efficiency should be < 1
Cycle time - Influenced by a series of constraints
Effective green time\ |
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HIOCC Algorithm - Queue protection#
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If detector is detecting for >2seconds send signal for reduced speed limit
slow long vehicles can trigger false alarms\ |
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Improvements to smart motorways#
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Convert dynamic hard shoulder sections to all-lane running
Improve spacing of emergency areas
more communication with drivers improving public information
displaying emergency areas on stanavs
updating highway code for more guidance\ |
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Incident Management#
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Incedent detection and verification
initial response and access
scene management
network restoration\ |
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Issues with dynamic hard shoulder#
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Time taken and resources used to open and close hard shoulder
cost of maintenance of technology infrastructure
capital cost of physical infranstructure
design issues around junctions and viaducts\ |
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Junction Layout#
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Must be safe, efficient and fair\
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Lane#
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Individual line of traffic containing one or more movements\
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Limitations of ramp metering#
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Congestion problems causes by a downstream bottleneck
reduction of road capacity because less lanes available
traffic queueing back from an off slip and blocking a lane on the main carriageway
roadworks/ accident resulting in closure of lanes\ |
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Limitations of VA signals#
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Only max green times optimised difficult to determine set of maxima appropriate at all times
Dominant flow on one arm can result in an inefficient junction as delays on other arms may be excessive\ |
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Low traffic neighbourhood#
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Scheme where motor vehicle traffic in residential streets are reduced
traffic routed elsewhere\ |
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Microporcessor Optimised Vehicle Actuation (MOVA)#
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Well suited for
High traffic flow
sites experiencing capacity difficulties under VA control with congestion on one or more approaches
sites with high speed approches and/or red compliance problems
where additional capacity is required to allow ped facilities or safer staging structure
Benefits from MOVA
13% delay saving compared to VA
2-3% capacity improvement\ |
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Motorway features#
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multiple lanes in each direction separated by central barrier
minimum radii curves - reduce lateral force and maximise sight lines
maximum gradients
grade separated interchanges as far as possible
no frontage access
emergency stopping lanes
no ped, slow moving vehicles, animals\ |
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Motorway Management Systems#
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Ramp metering
Queue protection and congestion management
Dynamic hard shoulder operations
Smart motorways\ |
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MOVA delay minimisation#
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If no links are over saturated then abs min green time\
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MOVA Modes#
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Under saturated: queue clears in every lane cycle, MOVA minimises delay
MOVA compares the benefits (reduction in delay and number of stops on current stage) of extending the green on the current stage against the disbenefit (increased delay on other stages). If benefits exceed the disbenefits then extend green otherwise change to next stage. Maximum green tim
Over saturated: Queue in any lane of link doesnt clear
MOVA determines oversaturation as occuring when
Estimated queue of vehicles exceeds preset value
If the IN vehicle detected has been detecting continuously for longer than a preset value
MOVA will seek to maximise capacity\
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MOVA set up data#
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The grouping of individual traffic lanes into “links” that have a common movement, e.g. straight ahead, ahead and left, right turn only.
• Distances from each detector to the stop-line
• Saturation flow information
• Typical cruise speed of traffic = 15th percentile speed of vehicles on the approach after the initial queue has finished discharging.
• Lane weighting- factors: to allow higher priority to be given to clearing the queue in some lanes when the junction is operating in oversaturated mode.
For each link,
• the signal stage(s) during which the link receives green,
• minimum green time,
• whether it is a traffic or pedestrian link
• stop penalty ( converts “stop” to a delay in seconds)\ |
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Movement#
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The trip executed between an origin and a destination within a junction\
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Pedestrial corssings at junctions#
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Require separate phase
Full green synchronised crossing period
or
movement can be with traffic\
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