Civil Engineering Reference
In-Depth Information
Table 17.2
Roundabouts design entry speeds and typical inscribed circle diameters
Type of roundabout
Recommended maximum entry speed (MPK)
Mini-roundabout
25
Urban compact
25
Urban single lane
35
Urban double lane
40
Rural single lane
40
Rural double lane
50
Type of roundabout
Circle typical design vehicle
Common inscribed circle
diameter range (ft)
Mini-roundabout
SN-30
45
-
90
Single-lane roundabout
WB-40
90
150
-
WB-50
105
-
150
WB-67
130
-
180
Multi-lane roundabout
2 lanes
WB-50
150
220
-
WB-67
165
220
-
Multi-lane roundabout
3 lanes
WB-50
200
-
250
WB-67
220
300
-
Source
Reference [
10
]
Effectiveness
Single-lane roundabouts have been reported to carry up to 25,000 vehicles per day.
Two-lane roundabouts have been reported to carry 40,000
50,000 vehicles per day
-
[
6
].
A growing body of literature indicates that roundabouts can reduce delay,
increase capacity, and improve safety. Cambridge Systematics [
2
] reports that:
in 38 cases reviewed, modern roundabouts reduced delay, increased capacity,
and increased safety.
in three roundabouts in Kansas, delay decreased from 13 to 23 %.
in 10 intersections with ADTs between 14,000 and 41,000 with signal control,
could have experienced lower average peak hour delays between 17 and 92 %,
had they been constructed as roundabouts.
17.2.2.5 Arterial Connections to Freeways
In many urban areas congestion is common along arterial roadways in the vicinity
of ramps to and from freeways. This is the result of (1) heavy traf
c volumes on the
arterial street and (2) heavy left turns (sometimes overlapping along the roadway)
