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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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Pedestrian crossings#
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Zebra - ped right of way, no signal control
Pelican - finds gap in traffic and provides res, otherwise wait until max green time
Puffin - checks peds are still waiting after button is pressed, monitors crossing as well\
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Phase#
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A set of streams that receive identical signal indications at the same time\
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Phase allocation#
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Number of phases to be kept as small as possibles
if there are large no. phases then alternative measures to be considered, e.g ban right turn
phases grouped into stages in a signal plan\
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Queue protection and congestion management#
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Queue protection: Hazard, risk and mitigation
e.g
hazard - collision from driving into rear of queueing traffic
risk - likelihood increases as increasing traffic volumes leads to queues forming
mitigation - increase driver awareness, lower vehicle speed\
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Ramp Metering#
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Regulates flow from slip road onto main carriageway at level that maximises through put on main carriageway
Flow breakdown becomes likely as traffic density slowly builds up at the merge
if critical capacity is exceeded flow breaks down\
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Ramp metering process#
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image/flowchart\
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Ratio of inflow to outflow#
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If inflow is > outflow queue forms
junction needs to be changed if this is the case e.g extra lane\
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Regression to mean#
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Random fluctuations in crash number periods with high number may be followed by numbers for the same area
This is because of the regression t mean phenomenon.
results in over-estimations of the effectiveness of an intervention intented to imrpive safety; so needs to be looked at for many years for consistency\
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Road safety statistics#
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Number - abs value to give idea of overall scale
Normalised by population - per 1mil
Normalised by number of vehicles per 1mil
Normalised by distance travelled per billion vehicle kilometres\
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Safe System Principles#
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Problem - Manage energy in crashes preventing fata and serious casualties
Goal - Zero fatal and severe injuries
Planning approaches - Proactively target and treat risk; systematic approach to build a safe road system
Causes of the problem - People make mistakes
Ultimate responsibility - Shared responsibility by individuals with system designers
How it works - Multiple element of a Safe System combine so that if one part fails others can provide protection\
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Saturation flow, S#
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Max rate of departures across a stop line for an individual stream\
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SCOOT#
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Split - green time split
cycle - traffic signal cycle time
offset - linking of traffic between signals
optimisation
technique\
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SCOOT concepts#
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short term prediction of flows
fast responce
incremental changes
dynamic signal timings
online traffic model\
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SCOOT optimisers#
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Split - optimising node green time split between each signal stage
Offset - optimising linking between nodes
Cycle - Minimising network traffic delay\
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SCOOT traffic model#
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Constructs a smoother traffic cyclic profile flow for each link
Stop line arrival profile made by offsetting cyclic flow profile by the link journey time Flowchart\
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Selecting speed limits#
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History of collisions
road geometry and engineering
road function
composition of road users
existing traffic speeds
road environment\
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Side - fire - radar#
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Single device covers multiple lanes, monitors traffic flow in each
can be blocked by taller vehicles\
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Signal control objectives#
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Safety
efficiency
management - prioritise certain vehicles e.g busses, trams
policy - e.g provide more space for pedestrians and cyclists\
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Signal control strategies#
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Fixed time (offline)
Online - program reacts to changing traffic conditions:
Vehicle actuated - detection of vehicles at stop line (MOVA) Dynamically controlled - signal times determined centrally based on date, time, traffic flow, density speed e.g SCOOT\
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Signing and Lining#
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Primary method for highway authorities to convey regulations, instructions and guidance
Major maintenance task
Major contribution to road safety\
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Smart motorway operations#
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designated emergency areas built
Maintenance issues
mitigation against hazards caused by incedents
time to repair\
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Smart motorways - All Lanes#
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Through junction running - Extending the dynamic use of hard shouldet through the junction
Controlled all lane running - converting the hard shoulder permanently to a running lane with speed control access across all lanes\
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Speed limits#
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Evidence led and self explaining to reinforce peoples assessment of what is a safe speed to travel
key source of information to road users; indicate the nature of risks for themselves and others on the road\
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Speed management measures#
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Visual clues
chicanes
speed humps
technology and psychology\
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Speed studies#
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Establish parameters for traffic operation and control
evaluate the effectiveness of traffic control devices
speed vs safety\
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Speed study methods#
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Road detectors
radar based
inductive loops
cameras\
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Stage#
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The period during which signals are not changing and a set of phases have green\
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Stopped vehicle detection#
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Radar identifies objects that slow down/stop and alerts are sent to the control room
cctv personnel verify presence of stopped vehicle and set signals to close lanes, reduce speed and dispatch recovery teams\
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Stream#
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Smallest set of movements that form a single queue comprising of one or more lanes\
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Temporary traffic management#
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safety zones and necessary working space
number and width of temporary traffic lanes; hard shoulder use?
contraflow
temp speed restrictions
arrangements for access to site\
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Temporary traffic management design principles#
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provision of clear and early warning of obstructions in the highway;
optimisation of road space and the provision of an adequate safety zone and working space at works locations;
clear directions relating to decisions/actions required from road users;
minimisation of potential conflict between road users, and between road users and road workers and their operations;
credibility of traffic signs and temporary requirements; and
speed limits and restrictions appropriate for the temporary highway geometry and safety features.
to maximise the safety of the workforce and the travelling public.\
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Time to repair - Smart motorways#
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Fault triage - what is wrong
Site access - get to the problem location
Fault repair and recomission - get technology working
Traige
tech that self-tests and reports errors
remote access via comms network to investigate
prioritise faults - not all reported faults prevent operation
Access
control of roadspace - need for temp traffic management
night time working
impact on other planned work
time to repair must be short enough to minimise risk of a hazard that cannot be fully mitigated\
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Vehicle actuation between limits#
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Use survey data to identify maximum flows
design signal plan
determine min green times and inter green periods
apply optimisation calculations to determine max cycle time and green time
no traffic means junction runs at min green times
at high flows junction operates at optimised max green times
detection system installed to control timings between periods\
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