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

Lec 26, Ch.7, pp.235-249: Islands and turning roadways (Objectives). Know the functions of islands Be familiar with minimum sizes of islands and and island edge treatments Learn how to determine minimum pavement widths of turning roadways

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

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  1. Lec 26, Ch.7, pp.235-249: Islands and turning roadways (Objectives) • Know the functions of islands • Be familiar with minimum sizes of islands and and island edge treatments • Learn how to determine minimum pavement widths of turning roadways • Details of turning roadway design (horizontal and vertical alignments, etc.) will be discussed in CE561. Here, just be aware that topics discussed in this book are the starting point of turning roadway design.

  2. What we discuss in class today… • Types of islands • Functions of islands • Minimum sizes and treatments of islands • Classification of turning roadways • Dealing with off-tracking • Method for determining minimum pavement widths for turning roadways

  3. 3. Islands Formed by Pavement Edges (Usually unpaved grass islands) Types of islands 1. Curbed Traffic Islands (barrier-type or mountable curbs) 2. Traffic Islands Formed by Pavement Markings (flushed islands)

  4. 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 intersection Look carefully. Divisional islands do NOT direct turning movements.

  5. Functions of traffic islands (2) Refuge islands: Provide refuge for pedestrian These islands must be wide enough to protect pedestrians.

  6. 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.

  7. 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 Min. 12 ft., but 15 ft. recommended • AASHTO recommends: • Min. about 50 sq. ft for urban intersections • Min. about 75 sq. ft for rural intersections • Min. about 100 sq. ft recommended Islands may be curbed or uncurbed. Curbs can be a barrier type or mountable type.

  8. 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.

  9. 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. Without shoulders For safety purpose

  10. Location and treatment of approach ends of curbed islands (2) Without shoulders

  11. Minimum pavement widths of turning roadways at at-grade intersections (3 Cases) Turning roadway

  12. 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. U is the track width that includes off-tracking effect. L u = normal track width RT = turning radius of the outer front wheel. L = wheelbase u Use of the Pythagorean theorem.

  13. 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

  14. And in a table format… (Table 7.4)

  15. 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.

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

  17. Design widths of pavements for turning roadways (Then make necessary modifications) For larger vehicle combinations, compare the widths obtained from Table 7.4 with the widths obtained from Table 7.3, and take larger values. (Review Example 7-1)

  18. In reality multiple types of traffic conditions are considered… See Table 7-6 See Fig. 7.17

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