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.
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

1. Curbed
Traffic
Types
of Islands
islands
(barrier-type or mountable curbs)
2. Traffic Islands
Formed by Pavement
Markings (flushed
islands)
3. Islands Formed by
Pavement Edges
(Usually unpaved
grass islands)
Functions of traffic islands (1)
Channelized islands:
Control and direct traffic
Look carefully. Divisional islands do NOT
direct turning movements.
Divisional islands: Divide
opposing or samedirectional traffic streams
and alert the drivers about
the intersection
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
Min. 12 ft., but
15 ft.
recommended
Islands may be curbed or uncurbed.
Curbs can be a barrier type or
mountable type.
AASHTO recommends:
 Min. about 50 sq. ft for
urban intersections
 Min. about 75 sq. ft for
rural intersections
 Min. about 100 sq. ft
recommended
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
v
Z
R
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 offtracking effect.
U  u  RT 
L
u = normal track width
RT = turning radius of the outer front
wheel.
L = wheelbase
RT 
R
2
T
 L2

u
Use of the Pythagorean
theorem.
L
RT
R
2
T
 L2

R
2
T
 L2

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 semitrailer 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)
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)
In reality multiple types of traffic conditions are
considered… See Table 7-6
See Fig. 7.17