Chapter 9 Questions to Ask Yourself
Intersection Design (1): Types and Examples of Intersections: p.9-1 – 928
General Design Considerations and Objectives
Pages 9-1 – 9-8
Q1. State the main objective of intersection design.
Q2. What are the five basic elements that should be considered in intersection design?
List the five elements and be ready to talk about the items listed under each element.
Q3. Define an intersection by both its functional and physical areas. Sketch both areas
Types and Examples of Intersections
Pages 9-8 – 9-20
The Greenbook illustrates 3-leg, 4-leg, and multi-leg intersections with plain basic, flared,
and channelized types. State the benefits of moving from plain basic to channelized types.
Q4. State the difference between flared and channelized treatments at intersections.
Modern Roundabouts
Pages 9-21 – 9-25
Q5. What are the difference between traffic circles (or rotaries) and modern
roundabouts?
Q6. What are typical inscribed circle diameters used for roundabouts so far?
Q7. Given a sketch of a single-lane roundabout, be ready to name its geometric elements.
Q8. What are the benefits of roundabouts compared to regular signalized intersections?
Q9. What are the issues (problems) of roundabouts?
Q10. What are the benefits of having flared entry to the circle?
Alignment
Pages 9-25 – 9-27
Q11. What would be the angle range you want to use to intersect two roads?
Q12. When realignment of the minor road is planned, what kinds of items should you
consider?
Profile
Pages 9-27 – 9-28
Q13. What would be the targeted grade near intersections to reduce the effect of grade on
vehicle performance?
Q14. How do you adjust the grades of the major and minor roads at intersections?
Intersection Design (2): Types of Turning Roadways: p.9-55 – 9-94
General
Pages 9-55
Q1. There are three typical types of right-turning roadway designs at intersections. What
are they?
Minimum Edge-of-Traveled-Way Designs
Pages 9-55 – 9-55
Q2. Define the “angle of turn” as used in Table 9-15: Edge-of-Traveled Way Designs for
Turns at Intersections and Table 9-16: Edge-of-Traveled Way Designs for Turns at
Intersections (continued).
Design for Specific Conditions (Right-Angle Turns)
Pages p.9-80 – 9-94
Q3. Three design vehicles and their turning characteristics and corner designs are
discussed in the Greenbook. Be ready to explain particular characteristics of their turns
and minimum dimensions of corners.
Q4. What is the impact of oblique turns on intersection design?
Q5. What is the effect of curb radii on pedestrians?
Intersection Design (3): Types of Turning Roadways: p.9-94 – 9-122
ISLANDS: General Characteristics & Types of Islands
Pages 9-94 – 9-95
Q1. Several purposes of channelizing islands are presented in page 9-94 and 9-95. What
are they?
Q2. Three primary functions of islands are: __________________, ________________,
and ____________________.
Q3. What kinds of islands are channelizing islands?
Q4. What kinds of islands are divisional islands?
Q5. Can you identify refuge islands in this community?
Island Size and Designation & Island Delineation and Approach
Treatment
Pages 9-95 – 9-106
Q6. What’s the typical minimum size of an island?
Q7. Islands are grouped into three groups in terms of the design. What are they? (p.9-99)
Q8. How are islands delineated?
Q9. What would be the difference between the design of islands for urban streets and
rural roadways?
Turning Roadways with Corner Islands
Pages 9-106 – 9-122
Q10. State the relationship between the turning roadways and island sizes.
Q11. Table 9-18 Typical Designs for Turning Roadways
Intersection Design (4): Intersection Sight Distance: p.9-28 – 9-35.
Sight Triangles
Pages 9-28 – 9-32
Q1. What are the two types of clear sight triangles considered in intersection design?
Q2. Study Figure 9-15 Intersection Sight Triangles carefully and be able to explain the
difference between the two types of clear sight triangles questioned in Q1.
Q3. What is the basic assumption in determining the extent of clear sight triangles? (p.930, 2nd paragraph)
Q4. What are the key values used in identifying sight obstructions within sight triangles?
Intersection Control
Pages 9-32 – 9-54
Q5. The recommended dimensions of the sight triangles vary with _____________.
Q6. There are six control cases for determining sight triangles. What are those controls?
Case A – Intersections with No Control
Q7. For Case A what level of perception-reaction time is assumed?
Q8. Table 9-3 Length of Sight Triangle Leg – Case A – No Traffic Control shows the
length of leg for different design speeds. These values are generally shorter than the SSD
values shown in Table 3-1 Stopping Sight Distance. Why are the values in Table 9-3
shorter than those in Table 3-1?
Q9. Where the grade along an intersection approach exceeds ____ percent, the leg of the
clear sight triangle along that approach should be adjusted by multiplying the appropriate
sight distance from Table 9-1 by the appropriate adjustment factor from Table 9-4
Adjustment Factors for Sight Distance Based on Approach Grade.
Q10. If an approach to an uncontrolled intersection has a design speed of 30 mph and the
approach grade is +5% what would be the length of sight triangle leg for that approach.
Case B – Intersections with Stop Control on the Minor Road
Q11. For this case, departure sight triangles for intersection with stop control on the
minor road should be considered. What are the three situations discussed in this section?
Q12. What are the key assumptions of Case B1 – Left turn from the minor road?
Q13. ISD = 1.47Vmajortg is used for this case. Explain why Vmajor is used for the vehicles
coming from the minor road. What is tg?
Q14. Know how to adjust for the number of lanes and minor road approach grades. Table
9-5 Time Gap for Case B1.
Q15. Know how to get values from Table 9-6 Design Intersection Sight Distance and
Table 9-7 Intersection Sight Distance.
Q16. Summarize how to obtain the intersection sight distance for right turns. P.9-40.
Q17. How do we determine the intersection sight distance for crossing maneuvers? See
Table 9-7 Time Gap for Case B2 and Case B3, Table 9-8 Design Intersection sight
distance for Case B2 and Case B3, and Figure 9-18 Intersection Sight Distance for Case
B2 and Case B3.
Case C – Intersections with Yield Control on the Minor Road
Q18. What is the general principles used for Case C?
Q19. Explain Equation 9-2.
Q20. Know how to use Table 9-9 Case C1 – Crossing Maneuvers from Yield-Controlled
Approaches – Length of Minor Road Leg and Travel Times and Table 9-10 Length of
Sight Triangle Leg along Major Road – Case C1 - Crossing Maneuvers from YieldControlled Approaches, and Figure 9-19 Length of Sight Triangle Leg along Major Road
for Passenger Cars – Case C1 – Crossing Maneuver.
Q21. Explain assumptions used for Case C2 – Left- and Right-turn Maneuvers
Q22. Know how to use Table 9-11 Time Gap for Case C2 – Left or Right Turn, Table 912 Design Intersection Sight Distance – Case C2 – Left or Right Turn at Yield-Controlled
Intersections, and Figure 9-20 Intersection Sight Distance – Case C2 – Yield-Controlled
Left or Right Turn.
Case D – Intersections with Traffic Signal Control
Q23. What sight distance would be the most important one that we should check for Case
D?
Q24. What kind of departure sight distance can be used? (The Greenbook is not specific
on this.)
Q25. If the signal is to be placed on two-way flashing operation, what kinds of sight
distance triangles are used?
Case E – Intersections with All-Way Stop Control
Q26. What is the condition of sight distances for intersections with all-way stop control?
Q27. For what kind of condition is an intersection with all-way stop control suitable?
Case F – Left Turns from the Major Road
Q28. What is the basic assumption for determining sight distances for Case F?
Q29. Know how to use Table 9-13 Time Gap for Case F – Left Turns from the Major
Road, Table 9-14 Intersection Sight Distance – Case F – Left Turns from Major Road,
Figure 9-21 Intersection Sight Distance – Case F – Left turn from Major Road.
Effect of Skew
Pages 9-54 – 9-55
Q30. Summarize how to deal with effects of skew on intersection sight distances fro
various cases.
Intersection Design (5): Median Openings: p.9-140 – 9-155.
There are many recommended dimensions for median openings and different
methods to accommodate left turns and U-turns in these pages. Their dimensions
are important when you design these facilities. However, what is more important to
you would be that you know benefits and disbenefits of these options. Focus on these
issues as you read these pages.