Chapter 16: Introduction to intersection control

Explain why some sort of control is essential to allocate ROW at intersectionsExplain there is hierarchy of intersection control (Traffic signal is NOT a cure-all)Check sight distances available at an intersectionUse the warrants for stop and yield controlsEvaluate traffic signal warrants

Chapter objectives: By the end of these chapters the student will be able to:

Conflict points at unsignalized intersections

T intersection

Crossing = 3

Merging = 3

Diverging = 3

Total = 9

4-leg intersection

Total = 32

16.1 The Hierarchy of Intersection Control

Intersection Control Options: How much judgment can drivers safely exercise to avoid collisions? Three levels of control are available.

Level I Passive control – basic rules of the road apply No control Guide signs only Warning sings with or without guide signs

Level II Direct assignment of ROW to major street or rotational ROW YIELD control (roundabouts are in this category) Two-way STOP control All-way STOP control

Level III

Positive alternate assignment of exclusive ROW Traffic signals: 2-phase, multiphase Traffic control agent/officer

16.2 Level I Control: Basic rules of the Road

Primary prerequisite for safety under basic rules-of-the-road: Sight distance must be adequate for the driver before he is accountable for full responsibility for his action. The first thing you want to check is whether adequate SSD is available at the intersection. This must be satisfied before traffic volume concerns come into consideration.

bd

add

dbd

abadab

dabd

ab

adbd

ab

a

bd

ad

b

A

AB

BA

A

BA

BA

A

B

From the similarity of triangles (1-3-2 and 6-4-5):

“Visibility Triangle; Veh A is on minor street”

How do you determine dA and dB?

Rule 1: Both vehicles have at least one safe SSD to the collision point (AASHTO recommended),

p.447

Step 1: Assume that Vehicle A is located one safe SSD from the collision point. Usually the vehicle is considered to be on the minor road.

Step 2: Based on the assumed position, determine the actual location of Vehicle B when it first becomes visible. Call it dB(act)

Step 3: Rule 1 requires Vehicle B to have one safe SSD. Call it dB(min).

Step 4: If dB(act) dB(min), then adequate SSD for basic rules-of-the-road has been provided. Otherwise, violated and under Rule 1, not safe.

GS

tSd AAA 01.0348.030

47.12

bd

add

A

AB

GS

tSd BBB 01.0348.030

47.12

(min)

Rule 2: Vehicle A must travel 18 feet past the collision point in the same time that Vehicle B travels to a point 12 feet before the collision point, p.448 (Historically, another approach…)

1218

468.1

12

468.1

18

(min)

A

BAB

B

B

A

A

S

Sdd

S

d

S

d

This dB is used in Step 3 in Approach 1. The remaining steps are the same as Approach 1.

dA

dB

When dBact < Dbmin then operation with no control cannot be permitted.

Implement intersection control, using STOP- or YIELD-control, or traffic signals.

Lower the speed limit on the major street to a point where sight distances are adequate.

Remove or reduce sight obstructions to provide adequate sight distances.

Sample problem: Fig. 16.3

Step 1:

Step 2:

Step 3:

Step 4:

ftdA 5.196)35.0(30

)30()5.2)(30(468.1

2

ftd actB 4.25425.196

)5.196)(20()(

ftd

ftd

RB

RB

0.2981230

40)185.196(

3.300)35.0(30

)40()5.2)(40(648.1

)2(min,

2

)1(min,

dB(act) << dB(min,R1) and dB(min,R2)

Both rules were violated.

dA

dB

The sight triangle between Vehicle A and B fails to meet the criteria for safe operation under basic rules of the road.

Table 16.1: Warrants for two-way STOP sign

Because the STOP sign causes a substantial inconvenience to motorists, it should be used only where warranted. A STOP sign may be warranted where one or more of the following conditions exist:

A. Intersection of a less important road with a main road where application of the normal ROW rule would not be expected to provide reasonably safe operation.

B. Street entering a through highway or street.

C. Unsignalized intersection in a signalized area.

D. High speeds, restricted view, or crash records indicate a need for control by the STOP sign.

16.3 Level II Control: Yield and STOP Control16.3.1 Two-way stop control (Check the example in page 450)

http://mutcd.fhwa.dot.gov/HTM/2003r1/part2/part2b1.htm

Sample Problem: Figure 16.4

18ft = (10 ft from the edge of curb to the stop line) + (8 ft form the front bumper to the driver)

dA-STOP = 18 + dcl

dBmin = 1.47*Smaj*tg

Table 16.2: Warrants for YIELD sign

The YIELD sign may be warranted:

A. When the ability to see all potentially conflicting traffic is sufficient to allow a road user traveling at the posted speed, 85th percentile speed, or the statutory speed to pass through the intersection or stop in a safe manner.

B. If controlling a merge-type movement on the entering roadway where acceleration geometry or sight distance is not adequate for merging traffic operations.

C. At a second crossroad of a divided highway, where the median width is 30ft or greater. A STOP sign may be installed at the entrance to the first roadway of a divided highway, and a YIELD sign may be installed at the entrance to the second roadway.

D. At an intersection where a special problem exists and where engineering judgment indicates that the problem is susceptible to correction by use of a YIELD sign.

YIELD

16.3.2 Yield control

Table 16.3: Warrants for multiway STOP sign (more specific)

Multiway stop signs: Requires that all vehicles approaching the intersection stop before entering it. Used when the traffic volumes on all the approaches are about equal. Warrants are:

a. Where traffic signals are justified, the multiway STOP sign is used as an interim measure.

b. Crash problems susceptible to correction by a multiway STOP sign exist, like right- and left-turn collisions as well as right-angle collisions.

c. Minimum volumes: (1) at least 300 veh/h for any 8 hours of an average day on the major street, and (2) at least 200 units (peds, bikes, vehs combined/h for the same 8 hours in the minor street with an average vehicle delay 30 sec/h during the highest hour , but (3) if the 85th percentile approach speed of the major highway exceeds 40 mph, the min vehicular volume warrants are 70% of the above values.

d. Where no single criterion is satisfied, but where criteria B, C1, and C2 are all satisfied to 80% of the minimum values. Criterion C3 is excluded from this condition

16.3.3 Multiway-way stop control

16.4 Level III Control: Traffic Control Signals

While warrants and other criteria for STOP and YIELD signs are somewhat general in the MUTCD, warrants for signals are quite detailed.

16.4.1 Advantages of traffic signal control Provide for the orderly movement of traffic

Increase the traffic-handling capacity of the intersection if applied properly and physical layout is conducive

Reduce the frequency and severity of certain types of crashes, esp. right-angle collisions

Provide for continuous or nearly continuous movement at a definite speed when coordinated (by coordination)

Used to interrupt heavy traffic at intervals to permit other traffic, vehicular or pedestrian, to cross

http://mutcd.fhwa.dot.gov/HTM/2003r1/part4/part4c.htm

16.4.2 Disadvantages of traffic signal control

Excessive delay (when timing is improper)

Excessive disobedience of the signal indications (when red interval is improperly long) – may encourage red-light running

Increased use of less adequate routes as road users attempt to avoid the traffic control signal

Significant increases in the frequency of collisions (especially rear-end collisions)

Data requirements1. Traffic volumes in each approach,

at least 12 consecutive hours (24 hrs count preferred) containing the highest % of 24-hour traffic (include two peak periods)

2. 15-min counts by approach and movement classified by vehicle type during the 2 hours in the AM and PM peak periods (4 hours total)

3. Pedestrian counts in each crosswalk during the same 4 hours in item 2

4. Information on nearby facilities and centers serving the movement disadvantaged (young, elderly, or disabled)

5. Posted or statutory speed limit and/or the 85the percentile speed

6. A condition diagram

7. A collision diagram

8. For the two peak AM and two peak PM hours:

a) Vehicle-hours of stopped delay for each approach

b) Number and distribution of gaps

c) Speed limits

d) Pedestrian delays for at least two 30 minute peak pedestrian delay periods

e) Queue lengths on STOP-controlled approaches

16.4.3 Warrants for traffic signals

(See page 454 and 455.)

MUTCD 2000 has 8 warrants for a traffic signal. They are guides, not specs. Use professional judgments.

Warrant 1: Eight-hour vehicular volume (1a: minimum traffic volume; 1b: interruption of traffic; 1c: combination of warrants)

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

Warrant 8: Roadway network

Signals

Warrants

Signal warrant 1A: 8-hour volume

Min. vehicle volume: Principal factor is the intersection traffic volume. Must satisfy for each of any 8 hour of an average day.

May reduce the values by 30% if the 85th percentile speed on the major approach is greater than 40 mph or population is less than 10,000 (built-up area of isolated community).

Signal warrant 1BInterruption of continuous traffic: The volume requirements must be met for each of any 8 hours of an average day.

May reduce the values by 30% if the 85th percentile speed on the major approach is greater than 40 mph or population is less than 10,000 (built-up area of isolated community).

Signal warrant 1C

Combination of warrants: Only in exceptional cases. When none of them are satisfied but when the first two warrants of Warrant 1 are satisfied to the extent of 80% of the stipulated volumes.

Signal warrant 2: 4-hour volume

Check for each of any 4 hr of an average day

Signal warrant 3: Peak-hour volume

Justified where traffic condition during 1 hr of the day result in undue delay to traffic on the minor street. (factories, office park etc.)

Signal warrant 3B: delay

This warrant is satisfied when the delay during any four consecutive 15-minute periods on one of the minor-street approaches (one direction only) controlled by a STOP sign is equal to or greater than specified levels and the same minor-street approach (one direction only) volume and the total intersection entering volume are equal to or greater than the specified levels:

1. Delay experienced by minor approaches 4 vehicle-hours for a one-lane approach and 5 vehicle-hours for a two-lane approach.

2. Volume on the same minor approaches 100 vph for one moving lane of traffic, 150 vph for two moving lanes of traffic.

3. The total entering volume serviced during the hour equals or exceeds 800 vph entering volume for intersections with four or more approaches or 650 vph for intersections with 3 approaches.

This applies to cases in which STOP control is already in effect for the minor road. Cannot jump from no-contro or YIELD to signals.

Signal warrants (4,5,6)

Warrant 4: Pedestrians. The pedestrian volume crossing the major street At least 100 for each of any 4 hr or 190 during any 1 hr and there are fewer than 60 gaps per hr that are acceptable by pedestrians for crossing. AND, the nearest signal along the major street should be at least 300 ft away. See Table 16.6 for details.

Warrant 5: School Crossing. The frequency of occurrence of gaps and the lengths of gaps are inadequate for safe crossing of the street. The number of acceptable gaps is less than the number of minutes in that period, e.g., if the crossing lasts 30 minutes, there must be at least 30 occurrence of critical gaps for children the cross. The minimum number is 20 during the highest crossing hour. If met only this warrant, use a pedestrian-actuated signal.

Warrant 6: Coordinated System. Used when lights would not otherwise have been installed. Justifies a signal if it will help maintain a proper grouping (platooning) of vehicles and effectively regulate group speed. See Table 16.7 for details.

Signal warrants (7,8)

Warrant 7: Crash Experience. Less restrictive measures must be used before this warrant is used. 5 or more injury or property-damage-only accidents within a 12-month period, AND signal control is a suitable countermeasure. AND, the traffic and pedestrian volumes should not be less than 80% of the requirements of warrants 1A and 1B. See Table 16.8 for details.

Warrant 8: Roadway Network. When lights help to encourage concentration and organization of traffic networks. (1) The total existing or immediately projected volume is at least 1000 during the peak hour AND the 5-year projected traffic volumes satisfy the requirements of one or more of the Warrants 1, 2, 3, on average weekday, OR (2) The total existing or projected entering volume is at least 1000 vehicles for each of any 5 hr of a Saturday and/or a Sunday. See Table 16.9 for details.

16.4.5 A sample problem in application of signal warrants

We will walk through 16.4.5 in class