Transportation Test 2

what are interchanges designed to do

facilitate the smooth flow of vehicles, minimizing disruption and improving safety

“at-grade intersection”

refers to a junction or crossing of two or more roadways at the same level or grade, meaning that the roads meet and intersect without the use of bridges, tunnels or overpasses

Due to _ traffic flows, intersections are a major source of crashes and delays

conflicting

when does conflicts occur

when traffic streams moving in different directions interfere with each other

Why signalization

• potential reduction of some types of crashes (particularly angle crashes)

• provisions for pedestrian to cross the street

• provisions for the progressive flow of traffic in a signal-system corridor

• possible improvements in capacity and reductions in delays

Why not signalization

• poorly timed signal or one that is not justified can have a negative impact on the operation of the intersection by:

  • increasing vehicle delay

  • increasing vehicle crashes (rear-end crashes)

  • causing a disruption to traffic progression and encouraging the use of routes not intended for through traffic

traffic signals cost around $250,000 for a __ but this doesn’t include maintenance

typical intersection

At certain point, __ and _ frequency/severity reach a level that warrants the installation of a traffic signal

traffic volume; crash

These warrants are provided in the _

MUTCD

what does MUTCD stand for

Manual on Uniform Traffic Control Devices

warrant 1

eight hour vehicular volume

warrant 2

four hour vehicular volume

warrant 3

peak hour

warrant 4

pedestrian volume

warrant 5

school crossing

warrant 6

coordinated signal system

warrant 7

crash experience war

warrant 8

roadway network

warrant 9

intersection near a grade crossing

interval

a period of time during which all signal indications (greens, yellows, reds) remain the same for all approaches

cycle

one complete sequence (for all approaches) of signal indications (greens, yellows, reds)

phase

the sum of the displayed green, yellow, and red times for one or more movements that start and stop moving at the same time

all red time

all approaches have a red indication

cycle length

the total time for the signal to complete one cycle

movement

a directional descriptor, such as left, through, and right

protected movement

a movement which has the right-of-way and doesn’t need to yield to conflicting movements, such as opposing vehicle traffic or pedestrians

left or right turn movements that are protected are given _

a green arrow indication

permitted movement

a movement which must yield to opposing trafifc flow or conflicting pedestrian movement

left or right turn movements with a green full circle indication are _

permitted movements

traffic signal controllers

designed to operate in one or more of the following modes: pre-timed, semi-actuated, or fully-actuated

pre-timed

a signal whose timing (cycle length, green time, and so on) is fixed over specified time periods and doesn’t change in response to changes in traffic flow at the intersection

semi-actuated

a signal whose timing (cycle length, green time, and so on) is affected when vehicles are detected (by video or pavement embedded inductance loop detectors) on some, but not all, approaches

fully actuated

a signal whose timing (cycle length, green time, etc.) is completely influenced by the traffic volumes, when detected, on all approaches. These are most commonly used at intersections of two major streets and where substantial variations exist in all approach traffic volumes over the course of a day.

saturation flow rate

the maximum hourly volume that can pass through an intersection, from a given lane or groups of lanes, if that lane(s) were allocated constant green over the course of an hour.

lost time

the time during which an intersection is not effectively serving any movement of traffic. it is a combination of start-up and clearance lost times

effective green time

the time during which a traffic movement is effectively utilizing the intersection

effective red time

the time during which a traffic movement is not effectively utilizing the intersection

capacity

intersection approaches do not receive a constant green indication (as in the definition for saturation flow rate)

What increases with the number of phases

total lost time per cycle

What is a primary concern in signal timing ?

keeping the number of phases to a minimum

Because protected - turn phases add to _, they should be used only when warranted

lost time

decisions on whether to provide a protected left-turn phase are based on one or more of the following factors:

• volume

• delay

• queuing (spillover)

• traffic progression

• opposing traffic speeds

• geometry (# of left turn lanes, crossing distance, sight distance)

• crash experience

What will applying a single PHF determined for the intersection as a whole result in?

more reasonable analysis volumes

What is a critical lane group

for any combination of a lane group movements during a particular phase, one of these groups will control the necessary green time for that phase

In this case, what is the critical lane group for each phase?

the lane group with the highest ratio of vehicle arrival rate to vehicle departure rate. This quantity is referred to at the flow ratio and is designated v/s ( arrival flow rate divided by saturated flow rate).

Y_c

sum of flow ratios for critical lane

what is the assumed start up lost time

2 sec

what is the assumed clearance lost time

2 sec ( 1 sec of yellow time plus 1 sec of all red-time)

what is the assumed lost time for each phase

4 sec

2 methods for cycle length: Highway Capacity Method

minimum necessary cycle length

2 Methods for cycle length: Webster Method

optimum cycle length

C_min

minimum necessary cycle length in seconds (rounded up to the nearest 5-sec increment)

Xc

• critical v/c ratio for the intersection

• = close to 1, but not higher than 1

Why is the value of 1 not recommend for Xc

due to the randomness of vehicle arrivals, and it can result in occasional cycle failures

What does the minimum cycle length equation describe ?

the minimum cycle length necessary for the intersection to operate at a specified degree of capacity utilization. This cycle length does not necessarily minimize the average vehicle delay experienced by motorist at the intersection.

what do you use when you want to know the length of the cycle to minimize delay (sec)

Optimum Cycle length (C_opt) formula

How do you find Allocate Green Time

subtracting the total lost time from the cycle length, the remaining time can be distributed as green time among the phases of the cycle (no rounding)

What is a change interval

• yellow time (amber time)

• to warn a driver of the end of a green time

• given after green time (typically 3-5 sec)

What is a clearance interval

• all red time

• to clear off vehicles already in the intersection

• given after yellow time

Change Interval and Clearance Interval are designed to eliminate what zone?

dilemma zone

In urban areas and other locations where pedestrians are present, the signal-timing plan should be checked for its ability to provide adequate pedestrian crossing time. At locations where streets are wide and green times are short, it is possible that pedestrians can be caught in the middle of the intersection when the phases changes.

To avoid this problem, the minimum green time required for pedestrian crossing time should be checked against the apportioned green time for the phase. If there is not enough green time for a pedestrian to safely cross the street, the apportioned green time should be increased to meet the pedestrian needs.

When can the green time be increased to meet pedestrian crossing needs

when pushbuttons are provided at an intersection (for actuated control) and they are activated

Vehicles:

• size and weight

• turning radii

• acceleration

• braking

• autonomous

• systems

• design vehicle

Vehicle Performance: Acceleration

• start up at intersections

• entrance ramps

• passing maneuvers

Vehicle Performance: Deceleration

• stopping sight distance

• exit ramps

Vehicle Performance: Ascending and Descending Grades

• truck climbing lanes

• runaway truck ramps

Vehicle Performance: Traffic Analysis

• speed limits

• signal timing

• progression timing (acceleration)

Vehicle Performance: Highway Design

• stopping sight distance

• passing sight distance

• safety

• ramps (acceleration and deceleration lengths)

• maximum grades

what is tractive effort

force available at the roadway surface to perform work (move the vehicle). It depends on available horsepower of vehicle. It is expressed in lbs of force

what is resistance

all forces acting on a vehicle which impede its movements. It is expressed in lbs of force. Major terms: aerodynamic (drag, shape of vehicle), rolling (tire, roadway surface), and grade resistance (gravitational)

aerodynamic resistance

• negligible at low speeds

• overwhelming at high speeds

• units: lbs

• sorces

  • turbulence around the body (85%)

  • air friction (12%)

  • air flow through components (3%)

rolling resistance

• sources:

  • tire deformation (90%)

  • pavement penetration (4%)

  • friction, other sources (6%)

• factors influencing sources:

  • tire inflation, temperature, speed

grade resistance

• gravitational force resisting a vehicles motion

• since highway grades are usually small, Sin 0_g= tan 0_g

Forces acting on a vehicle

F = ma + Ra+ Rrl + Rg

available tractive effort

force available to overcome resistance will be governed by engine or tire/pavement interface

What does the amount of tractive effort available to move a vehicle depend on?

tractive effort generated by the vehicle engine and maximum tractive effort available at the roadway surface

When will the vehicle’s wheels spin

if the engine generated effort is greater than maximum available

Maximum Tractive Effort

no matter how much force a vehicle’s engine makes available at the roadway surface, there is a point beyond which additional force results in the spinning of the tires

what does the coefficient of road adhesion depend on?

roadway conditions and vehicle characteristics (presence of ABS)

how does an engine work?

the engine creates rotational power which transfers to the wheels to move the car (done by transmission or drivetrain)

Common measures of engine performance:

• Torque M_e (work output- the twisting moment): ft-lb

• Power hp_e (torque/time: rate of engine work): hp

  • 1 hp = 550 ft-lb/s

Problem with gasoline and diesel engines:

• most tractive effort is needed at lowest speeds for acceptable vehicle acceleration

• highest torque/tractive effort provided is developed at fairly high speeds

Solution to gasoline and diesel engines:

gear reduction ratio

• ex: 4:1 means engine turns 4 times for every time the wheel turns

When are maximum braking forces developed?

when tires are at point of impending slide (max. friction)

what happens when the tires begin to slide (the breaks lock)

a significant reduction in road adhesion results

what do cars have to avoid locked condition

antilock braking systems (ABS)

Applications of Braking in Highway Engineering

stopping sight distance (SSD)

Applications of Braking in Highway Engineering: Placement of Warning Signs

MUTCD

Applications of Braking in Highway Engineering: Roadway Design Elements

• length of deceleration lanes

• ramps

Applications of Braking in Highway Engineering: Operations

intersection signal timing

Applications of Braking in Highway Engineering: Crash Avoidance Systems

pedestrian avoidance systems in autonomous vehicles

total stopping distance / stopping sight distance =

theoretical or practical stopping sight distance plus the distance traveled during perception/reaction

Sight Distance (SD)

length of the roadway ahead visible to drivers

• must allow driver to perceive, react, stop, change speed, swerve, etc. when necessary

Stopping Sight Distance (SSD)

the minimum distance that will allow the driver to come to a complete stop to avoid striking a stationary object in its path

Breaking Distance (theoretical or practical stopping distance)

distance traveled by a vehicle before coming to stop after applying brakes

Perception-Reaction Distance

distance traveled by the vehicle during the perception-reaction time