Lec 14, Ch. 15: Introduction to intersection control (Objectives)

Understand why some sort of control is essential to allocate ROW at intersectionsUnderstand there is hierarchy of intersection control (Traffic signal is NOT a cure-all)Learn how to check sight distances available at an intersectionLearn about the warrants for stop and yield controlsLearn how to check traffic signal warrants (No need to read pages 389-392 because it will not be in the 2000 MUTCD.)

What we cover in class today…

Counting the number of conflicts at intersectionsHierarchy of intersection controlMethod to check sight distances at intersectionsWarrants for STOP and YIELD signsWarrants for traffic signals

Conflict points at unsignalized intersections

T intersection

Crossing = 3

Merging = 3

Diverging = 3

Total = 9

4-leg intersection

Total = 32

Hierarchy of intersection controlIntersection 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 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

Assessing the viability of 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.

a

bd

ad

b A

B

From the similarity of triangles:

bd

add

A

AB

Rule 1: Both vehicles have at least one safe SSD to the

collision point

Step 1: Assume that Vehicle A is located one safe SSD from the collision point.

Step 2: Based on the assumed position, determine the 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-rule has been provided. Otherwise, violated and under Rule 1, not safe.

Gf

StSd AAA

30

468.12

bd

add

A

AB

Gf

StSd BBB

30

468.12

(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

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 Rule 1. The remaining steps are the same as Rule 1.

dA

dB

Sample problem: Fig. 15-5Step 1:

Step 2:

Step 3:

Step 4:

ftdA 7.129)35.0(30

)30()0.1)(30(468.1

2

ftd actB 6.29427.129

)7.129)(20()(

ftd

ftd

RB

RB

9.2081230

40)187.129(

1.211)35.0(30

)40()0.1)(40(648.1

)2(min,

2

)1(min,

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

Both rules were violated.

Warrants for YIELD sign

The YIELD sign may be warranted:

1. At the entrance to an intersection where it is necessary to assign ROW and where the safe approach speed on the entrance exceeds 10 mph

2. On the entrance ramp to an expressway where an acceleration lane is NOT provided.

3. At intersections on a divided highway where the median between the roadway is more than 30 ft. wide. At such intersections, a STOP sign may be used at the entrance to the first roadway of the divided highway and YIELD sign may be placed at the entrance to the second roadway.

4. Where there is a separate or channelized right-turn lane, without an adequate acceleration lane.

5. At any intersection where a special problem exists and where an engineering study indicates the problem to be susceptible to correction by use of the YIELD sign.

YIELD

Warrants for YIELD sign (cont)

Reference line: Approach speed of major street. If all 4 points lie above this line, the critical approach speed is in excess of 10 mph, and use of a YIELD sign is safe. Otherwise, use a STOP sign.

Warrant 1 for YIELD signs

b

a

Warrants for non-multiway 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:

1. Intersection of a less important road with a main road where application of the normal ROW rule is unduly hazardous.

2. Street entering a through highway or street.

3. Unsignalized intersection in a signalized area.

4. Other intersections where a combination of high speed, restricted view, and serious accident record indicates a need for control by the STOP sign.

Warrants for multiway STOP sign

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:

1. Total approach volume – not less than 500 vph for 8 hr of an average day, AND

2. the combined volume of vehicles and pedestrians from the minor approach – not less than 200 units per hour for the same 8 hours, AND

3. the average delay on the minor street – not less than 30 sec/veh during the maximum hour.

* May reduce the values by 30% if the 85th percentile speed on the major approach is greater than 40 mph.

Warrants for traffic signalsThe current MUTCD has 11 warrants for a traffic signal. They are guides, not specs. Use professional judgments. The 2000 MUTCD has 8 warrants (The first two warrants were combined in Warrant 1.) See the handout on the 2000 MUTCD.

Min. vehicular volume (1)

Interruption of continuous traffic (1)

Minimum pedestrian volumes (4)

School crossing (5)

Coordinated signal system (6)

Crash experience (7)

Roadway network (8)

Combination of warrants (1)

Four-hour volume (2)

Peak-hour delay (3)

Peak-hour volume (3)

Types of intersection control

Used mainly to separate turn lanes from through lanes. islands (raised or not raised) and channel lines

MUTCD 2000 has 11 warrants (grouped in 8 warrants) for a traffic signal. They are guides, not specs. Use professional judgments. Min. vehicular volume (1)

Interruption of continuous traffic (1)

Minimum pedestrian volumes (2)

School crossing (3)

Coordinated signal system (4)

Crash experience (5)

Roadway network (6)

Combination of warrants (1)

Four-hour volume (7)

Peak-hour delay (8)

Peak-hour volume (8)

Channelization

Signals

Data requirements1. Traffic volumes in each

approach, 16 consecutive hours containing the highest % of 24-hour traffic

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

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

4. 85th percentile speed on uncontrolled approach

5. A condition diagram

6. A collision diagram

7. Vehicle-seconds of delay for the same 4 hours in item 2, for each approach

8. Number and distribution of gaps in vehicular traffic for the same periods in item 2

9. 85% percentile speed on controlled approach, taken at a location not affected by the control, for the same period in item 2

10. Pedestrian delay for at least two 30-minute periods of an average weekday or other times needed, for the same period in item 2.

Signal warrants (1)1. 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 warrants (2)2. Interruption 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 warrants (3,4,5)3. Minimum pedestrian volume: 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.

4. 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. Use a pedestrian-actuated signal.

5. Progressive movement: 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.

Signal warrants (6,7,8)6. Accident 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 1, 2, and 3.

7. Systems: When lights help to encourage concentration and organization of traffic networks. (1) The toal existing or immediately projected volume is at least 1000 during the peak hour AND the 5-year projected traffic volumes satisfy the requirements of the following warrants: Warrant 1, 2, 8, 9, 11, 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.

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

Signal warrants (9. 4-hour volume)

Check for each of any 4 hr of an average day

Signal warrants (10. Peak-hour 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:

Delay 4 vehicle-hours for a one-lane approach and 5 vehicle-hours for a two-lane approach.

Volume 100 vph for one moving lane of traffic, 150 vph for two moving lanes of traffic, and 800 vph entering volume for intersections with four or more approaches or 650 vph for intersections with 3 approaches.

Signal warrants (11. Peak-hour volue)

Justified where traffic condition during 1 hr of the day result in undue delay to traffic on the minor street.