Vehicle Flow

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• Vehicle Flow

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Homework

• Ch 5 1, 2, 4, 7, 9

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Vehicle Flow

• How do vehicles operate in the system
• capacity
• speed
• headway
• density

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Time Space Diagrams

• Show vehicle location vs. Time
• one way or two way
• any type of transportation system
• used to find
• siding placement
• progression
• minimum headways
• Throughput

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Vehicle Following

• Spacing needed for stopping

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Deceleration

• 3 types of deceleration
• Normal dn 8fps2
• Emergency de 24fps2
• Instantaneous
• Initial spacing
• S vd v2/2df v2/2dl NL x0
• Safest spacing when lead vehicle decelerates
  instantaneously and following vehicle brakes at
  normal deceleration

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Example

• What is the safe initial spacing between two
  vehicles traveling 40 mph if the lead vehicle
  uses an emergency deceleration of 24 fps2 and
  the following vehicle normally decelerates at 8
  fps2? PR time 1.5 sec, vehicle length 20
  feet, x0 4 feet

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Flow Concepts

• 4 variables
• volume
• speed
• density
• headway

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Volume

• of vehicles passing a given point in a given
  unit time
• q n/T
• cars per hour
• does not tell you anything about speed or density

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Speed

• 2 types
• Space Mean Speed
• distance divided by time
• useful in determining vehicle flow
  characteristics
• Time Mean Speed
• Spot speeds
• Radar gun
• not useful except for tickets

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Example

• Si 2 miles, i 1 - 4
• v1 42 mph, v2 39 mph, v3 47 mph, v4
  50 mph
• Average V 44.5 mph
• m1 3.1 min, m2 2.8 min, m3 3.3 min, m4
  3.0 min
• Average 39.5 mph
• What is the difference, is it significant?

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Density

• Concentration
• vehicles per unit length
• cars/mile

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Headway

• Time or distance between two vehicles
• h 1/q gives the time headway
• h 1/k gives distance headway
• which is more useful?

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Relationships

• quk
• basic relationship
• Important points
• jam density
• jam speed
• max volume

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Relationships
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Relationships
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Relationships
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Relationships

• Shape of curves
• what do they tell us?

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Example

• The u-k relationship is
• u 2.6 0.001(k 240)2
• Find umax, jam density, and max capacity

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Shock waves

• Happens when traffic is forced to change speed
  either slowing down or speeding up
• shockwave
• Can move either forward or backward
• usw (qb qa)/(kb ka)
• a, b are 2 points of interest
• If shockwave is gt heading in direction of flow
• If shockwave is 0 gt stationary
• If shockwave is - gt moving against flow

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Example

• A line of traffic, moving at 30 mph and a
  concentration of 50 veh/mi is stopped for 30 sec
  at a red light. Calculate the velocity and
  direction of the stopping wave, the length of the
  line of cars stopped during the 30 sec of red,
  and the number of cars stopped during the 30 sec
  of red. Assume a jam capacity of 250 veh/mi

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Control of Vehicle Flows

• Control vehicles
• minimize accidents
• maximize effectiveness
• transit
• airports
• roadways
• Channelization most common control

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Control of Vehicle Flows

• Speed limits
• Control on Links
• signage
• lane width
• number of lanes
• headway rules

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Headway Rules

• RR - uses block system

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Headway Rules

• Aircraft
• Airports
• 1 aircraft on runway at any time
• separation distances
• 2.5 miles between aircraft
• 10 miles horizontal / 2000 ft vertical in air
• Control based on aircraft location
• Ground control for aircraft not on runway

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Capacity

• Capacity based on mode
• of vehicles per hour
• of passengers per hour
• arrivals, departures per hour

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Highways

• Capacity varies by road type
• Freeways
• no controls,
• Intersections
• traffic control
• 2 lane 4 lane roads
• lead vehicle
• All use Level of Service

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Determining Capacity

• Based on Roadway geometrics and traffic
  conditions
• For Freeways
• can determine ideal conditions and from that a
  maximum capacity

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Peak Hour Factor

• Measures demand peaking

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Level of Service

• LOS can be based on
• delay per vehicle
• speed
• service flow
• 6 LOS A - F
• A is best
• F is worst

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Freeway Capacity

• LOS based on
• density
• speed
• v/c
• For an LOS can find Maximum Service Flow (MSF)

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MSF

• MSF qmax(v/c) - ideal conditions
• SF q qmaxNfwFHV
• Table 4.3.3 for heavy vehicle factors
• Table 4.3.2 for width factors

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Capacity Restrictions

• Non recurring
• Recurring

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Example

• Ch 4 14

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Pedestrians

• LOS based on
• speed
• flow rate
• v/c ratio

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Bike LOS

• Based on amount of hindrance

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Airports

• Capacity depends on landside and airside
• LOS is based on delay and total time
• Why are airports becoming shopping malls?

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Rail

• Mass transit
• LOS based on pass/seat, space/ pass,
• both peak and off peak
• AMTRAK
• LOS based on wait time, comfort, pass/seat