**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

(Protected-Permissive)

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

Signs

Markings

Hardware

Be consistent with intersection geometry, volumes, speeds and pedestrian crossing requirements

Credit:

STM

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

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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

R

150

L

250*

550*

600

300*

280

*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