Traffic Operations Analysis Flashcards
Flow Rate
the number of vehicles that pass a point on a facility per unit of time
when the time unit is one hour or more, the flow rate is referred to as volume
Traffic Demand
the number of vehicles desiring to pass that point during the time period
PHF
relationship between hourly volume and maximum rate of flow within the hour
PHF = 1.0 - consistent flow
Lower PHF means more extreme peak
Speed
distance traveled per unit of time
Time Mean Speed
average speed of all vehicles passing a point over a time period
“spot speed”
Space Mean Speed
average speed of all vehicles occupying a given section of the roadway over a specific time period
Travel Time
the time required to travel a segment of given length
Running Time
total time the vehicle is in motion while traversing a segment of a given length
Density
number of vehicles in a given length of roadway or a lane
Headway
time between successive vehicles passing a point
Spacing
distance between successive vehicles passing a point
Equation for uninterrupted flow
v = S x D
flow = space mean speed x density
PHF Equation
PHF = hourly volume / (4 x peak 15 minute volume)
Capacity
ability of a transportation facility or service to meet the quantity of travel demanded by it
Describe uninterrupted flow
Vehicles are not interrupted by external factors such as traffic signals, stop/yield signs, or access points.
Freeways and long rural segments between controlled intersections.
Describe interrupted flow
Vehicle flow interrupted by factors such as traffic signals, stop/yield signs, frequent intersections/driveways.
Urban streets
Factors that impact capacity
- Lane widths
- Number and use of lanes
- Proximity of obstructions along edge of roadway
- Terrain
- Type of area (urban, suburban, rural)
- Vehicle mix (trucks/buses)
- Driver familiarity
When is No Control an option?
Total entering traffic less than 2000 units/day
Adequate approach sight distance
Three levels of traffic control
Uncontrolled
Stop or Yield
Signal
When is Yield or Two-Way Stop Control an option?
When right-of-way is assigned to the major street and the minor street traffic must stop or yield before proceeding
Total daily traffic on thru street less than 6000 veh/day
Adequate approach sight distance
3 or more crashes susceptible to correction by a stop sign
When should Multi-way Stop Control be considered?
As an interim measure where a traffic signal is justified
5 or more crashes in a 12m period are susceptible to correction
Minimum volumes
- 300 veh/hour over 8 hours on major street
- total entering volume 200 units/hr on minor street, with delay of 30 sec/veh during peak hour
- 70% of above if speed >40 mph
Steps for capacity analysis at signalized intersections
Determine the lane group flow rates
Determine the adjusted saturation flow rate
Determine proportion of traffic arriving on green
Determine capacity
Determine delay
Determine LOS
Access management
proactive management of vehicular access points to land parcels adjacent to raodways
Access management techniques
Access spacing
Driveway spacing
Safe turning lanes
Median treatments
Right-of-way management
Road diet
Reduction in width or number of lanes to reallocate that space for other uses, like bicycle lanes, pedestrian crossing islands, left turn lanes, or parking
System Interchange
Interchange connecting two freeways, with no at-grade ramp intersections
Service Interchange
Interchange connecting a higher functional class roadway to a lower functional class roadway, may have at-grade intersections
For capacity analysis, freeways are divided into what three parts?
Basic Segment
Weaving Area
Ramp Junctions
What is freeway capacity under base conditions?
2250-2400 passenger cars / hour / lane
Base conditions for uninterrupted flow - lane width
12’
Base conditions for uninterrupted flow - lateral clearance on right side
6’
Base conditions for uninterrupted flow - vehicle classification
All passenger cars
Base conditions for uninterrupted flow - driving population
commuter-type drivers (familiar with route)
Base conditions for uninterrupted flow - terrain
Level
Base conditions for uninterrupted flow - interchange density
2 miles or greater
Base conditions for uninterrupted flow - number of lanes in one direction
5 or more
Steps to calculate LOS
Flow Rate > FFS > Density > LOS
Free Flow Speed
speed of passenger cars measured under low to moderate flow rates
Relationship between speed, flow, density
v = S x D
D = v / S
Factors affecting operations at ramp junctions
Peak hour flow rates on mainline and ramp
Length of accel/decel lanes
Geometry, i.e. number of lanes
FFS of mainline and ramp
LOS in a weaving area is determined based on…
average running speeds
density of all vehicles in segment
Recurring Congestion
congestion that occurs repeatedly at a certain location during certain times of day
Non-recurring Congestion
congestion that occurs due to any traffic incident that impacts normal roadway conditions
Methods to increase freeway capacity
Widen to add lanes
Use shoulders as travel lanes
Reduce lane width to add lanes
Improvements to enhance bicycle safety
Paved shoulders
Wider outside traffic lanes if no shoulder
Bicycle lanes
Bicycle-safe drainage grates
Maintaining a smooth, clean surface
Micro-mobility
small, low-speed, human- or electric-powered transportation device
bicycles, scooters, electric scooters, etc.
Transportation (Travel) Demand Management
influencing people’s travel behavior in such a way that alternative mobility options are presented and/or traffic congestion is reduced
Site Specific TDM tools
Alternative Modes of Transportation (i.e. ride sharing, carpooling, bicycling)
Alternative Work Hours
Telecommuting
Area-wide TDM strategies
Growth management through urban design
Congestion pricing
Parking management
Auto restricted zones
Legislation
may include site specific techniques
Ramp Metering
reduces impacts of merging traffic on mainline traffic flow
How do traffic incidents impact the transportation system?
Reducing capacity
Increasing delay
Reducing safety
Types of managed lanes
HOV - preferential treatment for transit, carpools, etc.
HOT - combination of HOV and pricing strategies
Reversible Lanes - accommodate peak direction traffic demand
