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Fundamentals of Signal Timing and Design: Pretimed Signals

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on 26 March 2014

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Transcript of Fundamentals of Signal Timing and Design: Pretimed Signals

Fundamentals of Signal Timing and Design: Pretimed Signals
Pedestrian Signal Requirements
Pedestrians need sufficient time to cross intersections safely
Vehicular Signal Requirements
After developing an initial phase plan, the next step is determining the "timing" that accommodates vehicular demand
Sample Application:
Initial Signal Timing
Develop a phasing plan
Convert all turning movements to TVUs
Draw a ring-barrier diagram
Determine yellow and all-red intervals
Determine lost times per cycle
Determine desirable cycle length C
Allocate effective green time
Determine minimum pedestrian requirements and retime if necessary
Compound Signal Timing
Signal plans can be easily adapted to accommodate protected-permissive signal phasing
Treat the protected and permissive portions as separate phases
Use different TVU equivalents for each portion of the phase
Estimate the C and g treated the protected and permissive portions separately
The yellow transition between protected and permissive portions is treated as part of the green time for L turns
Development of Signal Phase Plans
Treatment of Left Turns
Permitted Left Turns
Protected Left Turns
Compound Phasing
Phase and Ring Diagrams
Common Phase Plans
Criteria for Left-Turn Phasing
Credit: STM, adapted from other sources
Types of Left-Turn Phasing
General Criteria for Left-Turn Phasing
Credit: STM
• Left-turn and opposing through volumes • Number of opposing through lanes
• Cycle length
• Speed of opposing traffic
• Sight distance
• Crash history
General Considerations
Use signal phasing to minimize collision risk by separating conflicting movements
Increasing the number of phases increases the total lost time but also increases left-turn saturation flow rates
Implement phase plans in accordance with MUTCD
Be consistent with intersection geometry, volumes, speeds and pedestrian crossing requirements
Typical vehicle and pedestrian movements at a four-leg intersection.
Example of a ring and barrier diagram showing the sequence of phases in time, from left to right.
Example of a phase and ring-barrier diagram for the intersection of two one-way streets
Basic Two-Phase Signalization
Each street receives one signal phase
All turns are permitted (exclusive turn lanes optional)
Left-turn and opposing through volumes do not create unreasable delays or unsafe conditions
Credit: Pearson Higher Education, 2011
Credit: STM
Ring-barrier diagram for two-phase signal
Exclusive Left-Turn Phasing
Need for left-turn protection indicated and provided by exclusive left-turn phase
Left-turn lane is provided
Phase plan can be modified to provide protected-permitted L on phases 2 and 6
Credit: Pearson Higher Education, 2011
Credit: STM
Ring-barrier diagram for exclusive left-turn phasing
Right-Turn Phasing
Multi-Leg Phasing
T-Intersection Phasing
Eight-Phase Actuated Control
Special Pedestrian Phasing
Leading-Lagging Green Phases
Separating left-turn phases can allow for different green times (and turning movement volumes
Direction with heavier turning volume is assigned green after exclusive left-turn phase (NEMA standard)
Can be modified to allow protected-permissive L turns in phases 4 and 6
Credit: Pearson Higher Education, 2011
Credit: STM
Ring-barrier diagram for protected lead-lag left turns
Actuated control allows phases to be skipped if no demand is detected
Exclusive L if demand is detected in both directions
Leading green if demand is detected in one direction
Combined through and R phase if no L demand is detected
Credit: http://ops.fhwa.dot.gov/publications/fhwahop06006/chapter_3p1.htm
Images: STM
Exclusive Pedestrian Phase
Pedestrian flows are significant enough to warrant exclusive green time
Can provide additional pedestrian safety by reducing right-turn conflicts
Additional vehicle delay due to lost time
Leading Pedestrian Interval
Useful to extend the pedestrian green time for a particular movement
Can improve safety by reducing conflicts with turning vehicles and establishing the presence of pedestrians at an intersection
T-Intersection Phasing, No LT Lane
Each approach requires its own signal phase
Increased delays for all vehicles and pedestrians
T-Intersection Phasing, LT Lane Provided
More efficient phasing plan
Intersection geometry allows several movements to use two of three phases
T-Intersection Phasing, Channelized Through Movement
Geometry allows for a signalization with continuous vehicle through movement
Used only if an overpass or underpass allows for pedestrian N-S movement
Credit: Pearson Higher Education, 2011
Credit: Pearson Higher Education, 2011
Four-phase signal plan required to accommodate all movements
Increased lost time, delay and reduced capacity
Utilize intersection design to eliminate five- and six-leg intersections whenever possible
Right turns are generally permitted
Protected right turn phasing used when pedestrian volumes are sufficiently high
Increase pedestrian delay
Right Turn on Red generally allowed
Reduced delay for right turning vehicles
Potential safety concerns
Credit: STM
Ring-barrier diagram showing a right turn overlap phasing, which assigns a protected right-turn phase to the complementary left turn movement on the intersecting roadway.
Context: General Phasing Criteria
Credit: STM
You are here
Change and Clearance Intervals
Lost Times
Default values in the HCM
Sum of Critical-Lane Volumes
Demand volumes must be converted to through-vehicle equivalents
Consider the ring diagram to determine critical-lane flows when phasing overlaps
Desired Cycle Length
Desired cycle length is computed using a default saturation flow rate (1,615 tvu/h)
Splitting the Green
Length of yellow/change interval
Length of all-red/clearance interval
significant pedestrian traffic
The time required for a vehicle to traverse one safe stopping distance at its approach speed
ITE Recommended Methodology
some pedestrian traffic
Calculating Approach Speed for Yellow and All-Red Intervals
The yellow interval equation utilizes the 85th percentile speed (speed in the numerator)
The all-red interval equation utilizes the 15th percentile speed (speed in the denominator)
If only the average speed is known, the percentile speeds are calculated as shown
If only the speed limit is known, the same value is used for both intervals
Example Problem
Given Data
Average approach speed = 35 mi/h
Grade = -2.5%
Distance from STOP line to far side of the most distant lane = 48 ft
Distance from STOP line to far side of the most distant crosswalk = 60 ft
Standard vehicle length = 20 ft
Reaction time = 1s
Deceleration rate = 10 ft/s^2
Some pedestrians present
Estimating percentile speeds:
Calculating the length of the
yellow interval:
Calculating the length of the all-red interval:
The maximum value, 1.5 s, is applied.
Estimating lost time per phase using the default values
Example Calculations:
Lost Time per Phase
The total lost time per cycle is the sum of lost times in each phase
Credit: Pearson Higher Education, 2011
Through Vehicle Equivalency Tables
Demand intensity per lane is found for each approach or lane group:
Total equivalent volume and equivalent volume per lane in each approach or lane group:
Determining Critical-Lane Volume
*Denotes the maximum volume for each movement
Example Problem
Assume a sum of CLV of 1,100 veh/h, total lost time per cycle of 12 s, PHF of 0.92 and a target v/c of 0.90
Find the desired cycle length
Need to divide available effective green time among various signal phases
Available effective green time in the cycle is found using the total lost time and cycle length:
Total effective green time is allocated to the various phases within the signal plan in proportion to the critical-lane volumes of each phase:
Example Problem
CLVs as previously calculated:
250 veh/h/ln for sum of phases
A1 and A2
550 veh/h/ln for phase A3
300 veh/h/ln for phase B
Total effective green time is computed:
The effective green times for the signal plan are estimated:
Due to the overlapping phases, need to estimate the effective green time allocated to phases A1 and A2:
The length of A2 is computed using the previously calculated totals for A1 and A3:
Finally, convert effective green times to actual green times:
*This step cannot be completed for the example due to the lack of yellow and all-red interval data
Green-time requirements for pedestrians in the HCM:
3.2 seconds is a start-up time for pedestrians, while the second term allocates green time based on pedestrian volume.
The minimum walk indication is estimated using the first and second terms:
For signal timing to be viable for pedestrians, green time in each phase must be compared with the time the agency wishes pedestrians to be in the crosswalk:
When the minimum condition is not met....
A pedestrian actuator may be provided to lengthen the next green phase
Preferred: retime the signal to provide the minimum pedestrian time needed for all cycles
Example Problem
Assume that pedestrians are allowed in the crosswalk during G, y and ar
Minimum pedestrian green time is computed:
Effective green time is computed for each signal phase:
The signal must be retimed for a Phase B of at least 30 s. Effective green time must be increased to:
Effective green time of phase A must be increased to maintain the original ratio:
Thus, the total cycle length after retiming is 100.6 s. Using pretimed control, a 110-s cycle length would be needed:
Relationship between pedestrian and vehicular timing
Credit: 2009 MUTCD
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