Traffic Operations Analysis

Question 1

Dfn Traffic Operations (1949 Burton W. Marsh)

Answer 1

Planning for and controlling the movement of vehicles and groups of vehicles over streets and highways for the purpose of attaining maximum efficiency and safety.

Definition has evolved to include all road users

Traffic operations analysis provides engineering - oriented basis for traffic operations and TSMO

Include the theory of traffic flow, study and evaluation of various strategies and designs and understanding of new modern surface transportation.

Question 2

Dfn Transportation Systems Management and Operations (TSMO or
TSM&O) FHWA

Answer 2

A set of strategies that focus on operational improvements that can maintain and even restore the performance of the existing
transportation system before extra capacity is needed

Question 3

What does TSMO look at?

Answer 3

System performance
(system level not individual project or corridor)

Question 4

The 4 key concepts relation to traffic flow

Answer 4

1. Volume and Demand
2. Density and Headway
3. Speed
4. Capacity

Question 5

What is flow rate?

Answer 5

The number of vehicles passing a point in a facilities per unit time.

Note: If the time is 1 hour or more then the flow rate is referred as volume.

Question 6

what is traffic demand?

Answer 6

The number of vehicles (or pedestrian) desired to pass a point during a time period.

Sufficient capacity - flow rate = demand
Not all were served - demand > capacity (flow rate reflect those served)

Question 7

What is Peak Hour Factor (PHF)

Answer 7

Is the relationship between the hourly volume and the maximum rate of flow within the hour.

Question 8

Speed

Answer 8

Is distance traveled per unit time.

Question 9

What are the 2 methods of calculating average speed?

Answer 9

1. Time Mean Speed (TMS)
2. Space Mean Speed (SMS)

TMS - The average speed of all vehicles passing a point over a period of time
SMS - The average speed of all vehicles occupying a given section of a roadway over a specified period of time

Question 10

What is travel time?

Answer 10

Is the reciprocal of speed.
The time required to travel a given length of a roadway.
Used to assess the performance of a transportation system.

Question 11

What is running time?

Answer 11

the total time when a vehicle is in motion while traversing a given length of a roadway.

Question 12

What is density ?

Answer 12

the number of vehicles in a given length of a roadway or lane. (veh/mi)

Question 13

what is headway?

Answer 13

Is the reciprocal of density.
Is the time between successive vehicles passing a point.

spacing is the distance between successive vehicles passing a point.

Question 14

The basic equation for uninterrupted flow is:

Answer 14

V = S * D

V - flow rate (veh/hr)
S - Space Mean Speed (mph)
D - Density (veh/mi)

Question 15

The 4 key areas/concept of HCM

Answer 15

1. Quantity of travel
2. Quality of travel based on user perceptions and expectations
3. Accessibility with which travelers can engage in desire activities
4. Capacity, the ability of transportation facility to meet the quantity of travel demanded

Question 16

2 Traffic Flow Concepts

Answer 16

1. Uninterrupted flow - vehicles are not interrupted by external factors such as traffic signals, stop or Yield signs or frequent driveway .
   (occurs on freeway and rural highways with long segments btw intersection controls)

2.Interrupted flow - vehicle flow is influenced by external factors such as traffic signals, stop, or yield signs, or frequent uncontrolled intersections or high volume driveways.
(occurs on urban streets)

Question 17

Intersection

Answer 17

area where two or more highways
join or cross, including the roadway and roadside facilities for traffic movements within the area

Question 18

Most common form of at-grade intersections include

Answer 18

Three -leg intersection (T-intersection)
Four-leg
Multi-leg intersection

Can be controlled or uncontrolled

Question 19

3 Level of intersection control include

Answer 19

1. No control device
2. – YIELD control
   – Minor street STOP control
   – All-way STOP control
3. Traffic signal

Question 20

Selection of appropriate level of control to use is determined by analysis of many factors including

Answer 20

traffic volume
roadway functional classification
crash history
roadway geometry
sight distance available

Question 21

Uncontrolled intersection control is used where

Answer 21

-traffic volumes are low (less than 2000 entering traffic )
-adequate sight distances are available for drivers to see vehicles on conflicting approaches. Adequate sight triangles must be available to all approaching motorists.
-Fewer than 5 right angle veh or ped crashes in 3 yrs

Question 22

yield intersection control is used where

Answer 22

According to the Manual on Uniform Traffic Control Devices (MUTCD), YIELD signs are warranted in the following situations:

-On approaches to through streets or highways where a full stop is not always necessary.
-At the second crossroad of a divided highway with a median width of 30 ft (9 m) or more, where STOP or YIELD signs may also be placed at the first roadway entrance.
-For channelized turn lanes separated by an island, even if adjacent lanes are controlled by signals or STOP signs.
-At intersections with special problems where engineering judgment suggests YIELD signs could resolve the issue.
-For merge-type movements where acceleration geometry or sight distance is inadequate, based on engineering judgment.

Question 23

STOP intersection control is used where

Answer 23

According to the MUTCD, STOP control on minor approaches should be considered when engineering judgment determines a stop is always required due to one or more of the following:

-Traffic volumes on the through street or highway exceed 6,000 vehicles per day.

-Restricted views necessitate stopping to observe conflicting traffic safely.

-Crash records show 3 or more correctable crashes within 12 months or 5 or more within 2 years, including right-angle collisions caused by failure to yield on minor approaches.

Question 24

Departure Sight Distance

Answer 24

Sight distance for minor-street STOP control is based on drivers’ gap acceptance.
The length of the sight triangle along the major street (ISD) depends on:
Traffic speed on the major street.
Accepted time gap (tg) ) for minor-street drivers.
AASHTO design policy assumes:
-The driver’s eye is 14.5 ft (4.4 m) from the near edge of the major street’s traveled lane.
Recommended time gap (tg):
-7.5 seconds for passenger cars turning left onto a two-lane, level-grade major road.
Time gap must be adjusted if conditions change.

ISD = 1.47 x V x tg

Question 25

All-way STOP intersection control is used where

Answer 25

*Multi-way STOP control is a safety measure for specific situations and should not be used solely to control speed. *It is appropriate when intersecting roadway volumes are roughly equal. *Conditions for installation per MUTCD: A) Interim measure: Where traffic signals are justified but pending installation. B) Crash history: Five or more correctable crashes (e.g., right-turn, left-turn, right-angle) reported in 12 months. C) Minimum volumes: 1) Major street: At least 300 vehicles/hour for any 8 hours of an average day. 2) Minor street: Combined vehicular, pedestrian, and bicycle volume of at least 200 units/hour for the same 8 hours, with an average delay of 30+ seconds per vehicle during the peak hour. 3) Speed adjustment: If the 85th percentile speed on the major street exceeds 40 mph, minimum volume warrants are 70% of the above. E) Partial criteria: If 80% of Criteria B, C.1, and C.2 are met, excluding C.3, a multi-way STOP may still be warranted.

Question 26

The capacity analysis procedure for vehicle traffic at signalized intersections involves several steps:

Answer 26

*Lane Group Flow Rates *Adjusted Saturation Flow Rate *Proportion Arriving During Green *Signal Phase Duration *Capacity and v/c *Delay *LOS Software packages are used and available eg HCS

Question 27

Determine adjusted saturation flow rate?

Answer 27

The "Lane Group" is the fundamental unit of analysis for each intersection approach. The Saturation Flow module calculates the adjusted saturation flow rate for each lane group. Base saturation flow rates: 1900 pcphgpl in larger metropolitan areas. 1750 pcphgpl in areas with populations under 250,000.

Question 28

Determine proportion arriving on green

Answer 28

Control delay and queue size depend on the proportion of vehicles arriving during green and red signals. *The proportion arriving on green is estimated based on: a) The proportion of green time for a lane group. b) The quality of signal system progression.

Question 29

Determine capacity and v/c

Answer 29

capacity for each lane group is calculated as: c = N x s x (g/C)

Question 30

Determine delay

Answer 30

control delay is calculated as a basis to determine level of service d = d1+ d2+ d3 Where:d = control delay (s/veh) d1 = uniform delay d2= incremental delay d3= initial queue delay

Question 31

Determine LOS

Answer 31

Level of Service (LOS) is based on average control delay per vehicle. *LOS can be expressed for: Each lane group. Each approach. The entire intersection. If the flow rate exceeds capacity, LOS is automatically “F”, regardless of average delay.

Question 32

Access management

Answer 32

Access management balances traffic movement and land access. It involves controlling: -Location, spacing, and design of traffic signals. -Local street connections and driveways. Goals: -Provide reasonable access to land developments. -Preserve safe and efficient traffic flow on roadways

Question 33

Access management tools

Answer 33

Good access management enhances safety and efficiency in the transportation network. It uses techniques to control access to highways, major arterials, and other roadways, including: a) Access Spacing: Increasing distance between traffic signals improves traffic flow, reduces congestion, and enhances air quality. b) Driveway Spacing: Fewer, more spaced driveways ensure orderly merging and reduce challenges for drivers. c) Safe Turning Lanes: Features like dedicated left/right-turn lanes, indirect left-turns, U-turns, and roundabouts maintain smooth through-traffic flow. d) Median Treatments: TWLTL and raised medians effectively regulate access and reduce crashes e) Right-of-way management: for future widenings, good sight distance, access location and other access related issues

Question 34

Traffic calming

Answer 34

Traffic calming uses design and measures on existing roads to: -Reduce vehicle speeds. -Improve safety for pedestrians and cyclists. Examples include: -Vertical deflections: Speed humps, speed tables, raised intersections. -Horizontal shifts and roadway narrowing to slow traffic and enhance the environment for non-motorists.

Question 35

Traffic Calming

Answer 35

Closures (e.g., median barriers) obstruct traffic to reduce cut-through traffic. Road Diets - Reduce lane width or the number of travel lanes, reallocating space for: -Bicycle lanes. -Pedestrian crossing islands. -Left-turn lanes. -Parking (serving as a pedestrian buffer). Benefits of road diets: -Shorter pedestrian crossings. -Room for pedestrian medians. -Safer bicyclist environments. -Reduced rear-end and side-swipe crashes. -Better speed compliance and crash severity reduction. Overall Impacts of road diets: - Reduce traffic speeds and collisions. -Improve safety for pedestrians and cyclists. -Encourage walking and cycling activity.

Question 36

Considerations for all users

Answer 36

Interruptible Flow Facilities: - Focus should extend beyond motor vehicles to include all road users. - Considerations must address pedestrians, cyclists, drivers, transit riders, and other modes of travel. - Ensure all users can travel efficiently and safely, regardless of their mode of transportation.

Question 37

Uninterrupted flow facilities

Answer 37

Include freeways, Built to the highest design standards, carrying the heaviest traffic volumes. rural roads LOS is based on relationship of Speed Flow rate Density

Question 38

Interchange Types:

Answer 38

1) System Interchanges: Connect two controlled-access highways or freeways. No at-grade intersections at ramp terminals. Examples: Directional and full cloverleaf interchanges, allowing continuous high-speed traffic flow. 2)Service Interchanges: Link higher functional class roadways to lower class roadways. May include at-grade intersections at ramp terminals. Examples: Diamond, partial cloverleaf, and single-point urban (SPUI) interchanges.

Question 39

Freeway Capacity (HCM):

Answer 39

Defined as the maximum hourly rate vehicles can traverse a point or section under prevailing conditions. Levels of Service (LOS): Measures operational conditions through speed, travel time, and driver comfort. LOS ranges from A (best) to F (worst).

Question 40

Freeways are divided into three sections:

Answer 40

1) Basic Freeway Sections: Outside ramp and weaving influences. 2) Weaving Areas: Where multiple traffic streams cross along the freeway. 3) Ramp Junctions where on or off ramps join the freeway.

Question 41

Base conditions for uninterrupted flow include:

Answer 41

*12 ft (3.6 m) lane width *No obstructions within 6 ft (1.8 m ) of the right edge of traveled way *Only passenger cars in the traffic stream *Driver population principally regular users *Level terrain with design that allows 70 mph (110 km/h) free flow speeds *Interchange density ≥ 2 mi (3.2 km) *≥ 5 lanes in one direction (urban freeways)

Question 42

LOS Determination for Freeways:

Answer 42

Density is the measure, calculated as flow rate divided by speed. Density Calculation Steps: 1) Determine flow rates: Start with ideal conditions, then adjust. 2) Determine free flow speed: Measured or adjusted for geometric design constraints. 3) Determine LOS: Use a chart with speed and flow rate inputs.

Question 43

Flow rates

Answer 43

Flow rates for capacity and service volumes on freeways are calculated by adjusting the base service volumes using factors that account for: a) Roadway geometry b) Driver population c) Heavy vehicles d) Hourly flow variation

Question 44

Free flow speed (FFS)

Answer 44

Free flow speed is the speed of passenger cars under low to moderate flow rates (up to 1,300 pcphpl). It can be determined in two ways: a) Field measurement: Using a spot speed study on a freeway section. b) Estimation: Based on physical characteristics of the roadway FFS = 75.4-flw-flc-3.22TRD^(0.84)

Question 45

Density calculation

Answer 45

D = V/S Density is the measure of effectiveness for LOS.

Question 46

Ramp junctions

Answer 46

Ramp junctions are points where traffic merges or diverges from a freeway. Key points include: - Diverging ramps: Rarely cause congestion unless there is traffic backup due to incidents or congestion at the ramp's intersection with a street. -Merging traffic: Vehicles entering the freeway at on-ramp locations seek gaps to merge into the traffic stream. -Operational effects: The primary impacts are on lanes 1 and 2, which are adjacent to the merge area, causing turbulence in the traffic flow.

Question 47

The level of service at ramp junctions is determined by

Answer 47

The density within the merge or diverge area of influence. Factors affecting operations at ramp junctions include: A) Peak hour flow rates (pcph) on both the freeway and ramps B) Length of acceleration or deceleration lanes at the ramps C) Geometric conditions: Single or multilane ramps, lane additions or drops, ramp geometry, and nearby ramps D) Free flow speeds of freeway and ramp traffic

Question 48

Weaving Sections

Answer 48

Weaving involves lane change maneuvers, and the HCM defines three types of weaving sections: Type A: Each weaving vehicle makes only one lane change to perform the weave. Type B: One weaving maneuver can be made without a lane change, while the other maneuver requires at most one lane change. Type C: One weaving maneuver can be made without a lane change, but the other maneuver requires two or more lane changes.

Question 49

Congestion on urban freeways can be categorized into

Answer 49

1) Recurring Congestion occurs repeatedly at specific locations and times of the day, typically due to high traffic demand exceeding roadway capacity. 2) Nonrecurring Congestion is caused by incidents that disrupt normal traffic flow. T

Question 50

Increasing freeway capacity by

Answer 50

Adding lanes. Methods to Increase Capacity: - Construct additional lanes. - Use shoulders as travel lanes (either full-time or during rush hours) for mixed traffic or HOVs. Impact of Using Shoulders as Travel Lanes: - Loss of shoulder's intended function as a refuge for stopped vehicles. - Potential increase in certain types of crashes.

Question 51

Impact of Traffic Incidents on the Transportation System:

Answer 51

a) Reduced Capacity: Incidents block lanes, decreasing available road space. b) Increased Congestion: Traffic builds up due to lane closures and slower speeds. c) Reduced Safety: Higher risk of secondary crashes and unsafe driving conditions.

Question 52

Capacity Reduction vs. Blockage:

Answer 52

Capacity Loss Greater Than Blockage: A small percentage of blockage can cause a larger reduction in capacity. - Example 1: One of two lanes blocked → 66% capacity reduction. - Example 2: Three of five lanes blocked → 80% capacity reduction. No Lane Blockage: Even incidents entirely within the shoulder can reduce capacity due to: - Traffic merging from blocked lanes into open lanes. - The gawker effect (drivers slowing to view the incident). - Resulting upstream congestion. Additional Impact: Emergency vehicles moving through traffic can further reduce capacity and cause disruption in the remaining lanes.