Vehicle Flow

Homework

• Ch 3 # 1,4, 7, 15

Vehicle Flow

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

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

Example• Canal System - Cohoes and Schenectady are on the Erie Canal

approximately 10 miles apart. There is a lock 3 miles from Schenectady. On the canal, barges cannot pass each other except in opposite directions ( the mules get tangled otherwise). The canal lock can raise or lower 1 barge in 20 minutes. it takes an additional 20 minutes to return the lock to its original position to handle another barge going in the same direction. Barges move at 5 mph. 3 barges can be accommodated at the lock either upstream or downstream until it is their turn to move through the lock. Barges leave Schenectady every 20 minutes and Cohoes every 30 minutes. Does the system work or should the schedule be revised?

Vehicle Following

• Spacing needed for stopping

Deceleration

• 3 types of deceleration– normal– emergency– instantaneous

Example

• Ch 3 # 2

Flow Concepts

• 4 variables– volume– speed– density– headway

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

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

Example

• Si = 2 miles, i = 1 - 5

• v1 = 42 mph, v2 = 39 mph, v3 = 47 mph, v4 = 50 mph

• m1 = 3.1 min, m2 = 2.8 min, m3 = 3.3 min, m4 = 3.0 min

• What is the difference, is it significant?

Density

• Concentration– vehicles per unit length– cars/mile

Headway

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

Relationships

Relationships

• q=uk– basic relationship

• Important points– jam density– jam speed– max volume

Relationships

• Shape of curves– what do they tell us?

Example

• Ch 3 # 8

Shock waves

• Happens when traffic is forced to change speed either slowing down or speeding up

• Can move either forward or backward

Example

• Ch 3, # 15

Pedestrians

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

Bike LOS

• Based on amount of hindrance

Examples

• Ch 4. # 1, 2

Homework

• Ch 4 #3,10, 12, 15, 16, 21,25,31, 36,37,48

Control of Vehicle Flows

• Control vehicles– minimize accidents– maximize effectiveness

– transit

– airports

– roadways

• Channelization most common control

Control of Vehicle Flows

• Speed limits

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

Headway Rules

• RR - uses block system

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

Capacity

• Capacity based on mode

• # of vehicles per hour

• # of passengers per hour

• arrivals, departures per hour

Highways

• Capacity varies by road type– Freeways

• no controls,

– Intersections• traffic control

– 2 lane & 4 lane roads• lead vehicle

• All use Level of Service

Level of Service

• LOS can be based on– delay per vehicle– speed– service flow

• 6 LOS A - F– A is best– F is worst

Determining Capacity

• Based on Roadway geometrics and traffic conditions

• For Freeways– can determine ideal conditions and from that a

maximum capacity

Peak Hour Factor

• Measures demand peaking

Example

• Ch 4. # 11

Freeway Capacity

• LOS based on – density– speed– v/c

• For an LOS can find Maximum Service Flow (MSF)

MSF

• MSF = qmax*(v/c) - ideal conditions

• SF = q = qmax*N*fw*FHV

• Table 4.3.3 for heavy vehicle factors

• Table 4.3.2 for width factors

Capacity Restrictions

• Non recurring

• Recurring

Example

• Ch 4 # 14

Airports

• Capacity depends on landside and airside

• LOS is based on delay and total time

• Why are airports becoming shopping malls?

Rail

• Mass transit– LOS based on pass/seat, space/ pass,

• both peak and off peak

• AMTRAK– LOS based on wait time, comfort, pass/seat