Lec 26, Ch.7, pp.235-249: Islands and turning roadways (Objectives)

Know the functions of islandsKnow the functions of islands Be familiar with minimum sizes of islands and and Be familiar with minimum sizes of islands and and

island edge treatmentsisland edge treatments Learn how to determine minimum pavement Learn how to determine minimum pavement

widths of turning roadwayswidths of turning roadways Details of turning roadway design (horizontal and Details of turning roadway design (horizontal and

vertical alignments, etc.) will be discussed in vertical alignments, etc.) will be discussed in CE561. Here, just be aware that topics discussed CE561. Here, just be aware that topics discussed in this book are the starting point of turning in this book are the starting point of turning roadway design.roadway design.

What we discuss in class today…

Types of islandsTypes of islands Functions of islandsFunctions of islands Minimum sizes and treatments of islandsMinimum sizes and treatments of islands Classification of turning roadwaysClassification of turning roadways Dealing with off-trackingDealing with off-tracking Method for determining minimum Method for determining minimum

pavement widths for turning roadwayspavement widths for turning roadways

Types of islands1. Curbed Traffic Islands (barrier-type or mountable curbs)

3. Islands Formed by Pavement Edges (Usually unpaved grass islands)

2. Traffic Islands Formed by Pavement Markings (flushed islands)

Functions of traffic islands (1)

Channelized islands: Control and direct traffic

Divisional islands: Divide opposing or same-directional traffic streams and alert the drivers about the intersectionLook carefully. Divisional islands do NOT

direct turning movements.

Functions of traffic islands (2)

Refuge islands: Provide refuge for pedestrian

These islands must be wide enough to protect pedestrians.

Island used for traffic calming

“Center Island Narrowing”

The motorist feels squeezed in a narrow lane and, feeling unsafe, slows down the speed. Slowing down reduce noise, also.

Minimum sizes of islands Need to be large enough to

“command attention” or be clearly visible and warn the driver

Need to be large enough to protect pedestrians

AASHTO recommends:

Min. about 50 sq. ft for urban intersections

Min. about 75 sq. ft for rural intersections

Min. about 100 sq. ft recommended

Min. 12 ft., but 15 ft. recommended

Islands may be curbed or uncurbed. Curbs can be a barrier type or mountable type.

Minimum sizes of islands (2)

Elongated islands or divisional islands:

Min. 20 to 25 ft

(100 ft min. for curbed divisional islands at isolated intersections on high-speed roads and preferably longer)

4 ft min. (absolute min. is 2 ft)

Avoid placing curbed divisional islands at isolated intersections on high-speed roads unless it is made clearly visible to the motorists. Hazardous when the vehicle hits the curb. Need signs and markers to warn the drivers. Basically you want to guide the vehicle gradually to the intersection.

Location and treatment of approach ends of curbed islands

Just take a look at examples in Figure 7-15 and 7-16 to get general ideas about location and treatments the curbed islands

This little space functions like a shoulder. Also it provides space for dirt and water.

For safety purpose

Without shoulders

Location and treatment of approach ends of curbed islands (2)

Without shoulders

Minimum pavement widths of turning roadways at at-grade intersections (3 Cases)

Turning roadway

Z and U in page 246

Z is an empirical values for the extra width allowance to compensate for the difficulty in maneuvering. v = design speed.

R

vZ

U is the track width that includes off-tracking effect. 22 LRRuU TT

u = normal track width

RT = turning radius of the outer front wheel.

L = wheelbase

L

22 LRT

22 LRR TT

TR

Use of the Pythagorean theorem.

L

u

If we determine pavement widths solely based on each design vehicle’s characteristics…

FA = 0.5 ft for passenger cars and 0 ft for trucks

And in a table format… (Table 7.4)

What are the traffic conditions? Traffic condition A: Passenger vehicles

are predominant, but this traffic condition also provides for the operation of an occasional large truck.

Traffic condition B: Proportion of SU vehicles warrants this vehicle type to be the design vehicle, but it allows for about 5-10% of the total traffic to be semi-trailer vehicles.

Traffic condition C: Proportion of semi-trailer (WB-40 or WB-50) vehicles in the traffic stream warrants one of these vehicle types to be the design vehicle.

Design widths of pavements for turning roadways (First find the basic width)

Design widths of pavements for turning roadways (Then make necessary modifications)

(Review Example 7-1)

For larger vehicle combinations, compare the widths obtained from Table 7.4 with the widths obtained from Table 7.3, and take larger values.

In reality multiple types of traffic conditions are considered… See Table 7-6

See Fig. 7.17