dr. essam almasri advanced traffic engineering 3. roundabout
TRANSCRIPT
Dr. Essam AlmasriAdvanced Traffic Engineering
Roundabouts are circular intersections with
1. Yield control on all entering vehicles
2. Channelized approaches
3. Appropriate geometric curvature to ensure travel
speeds in the roundabout of 30 mph (50km/hr) or
less
Background
Dr. Essam AlmasriAdvanced Traffic Engineering
Old type was called conventional roundabout
There are 3 types of roundabout:
Normal roundabout
It has a one – way circulating carriageway around a kerbed central island. The entries may or may not have flared approaches. (Preferred to be flared)
In Britain the central island is normally 4m or more in diameter and the entries are flared.
Mini roundabout
It has a one – way circulating carriageway around a flush or slightly raised circular marking less than 4m in diameter. The entries may or may not have flared approaches.
Double roundabout
It has two normal or mini roundabouts either contiguous (having a common border) or connected by a central link road or kerbed island.
Background
Dr. Essam AlmasriAdvanced Traffic Engineering
Mini roundabouts can be particularly effective in :
-Improving existing urban junctions that experience capacity and safety problems.
-It is only used when speed limit is < or = 48 km/h.
Double roundabouts have a number of special applications:
.at an awkward (uncomfortable) site such as a scissors junction
.at an existing staggered junction
.at overloaded single roundabouts.
.at junctions with more than four entries.
Background
Dr. Essam AlmasriAdvanced Traffic Engineering
Other types of roundabout:
-Signalised roundabout that has traffic signals installed on one or more of the approach arms.
-Grade separated roundabouts which has at least one entry road via an interconnecting slip road from a road at a different level, e.g. underpasses, flyovers.
Background
Dr. Essam AlmasriAdvanced Traffic Engineering
Data Requirements for Capacity calculation
1. Movement traffic flow (Left, Right, Through, U-Turn)2. Circulating flow3. Passenger Car Equivalent (PCE) 4. Geometric data
Dr. Essam AlmasriAdvanced Traffic Engineering
Capacity
Roundabout approach capacity is dependent on 1. the conflicting circulating flow and 2. the roundabout’s geometric elements.
Roundabouts should be designed to operate at no more than 85 percent of their estimated capacity. Beyond this threshold, delays and queues vary significantly from their mean values.
Dr. Essam AlmasriAdvanced Traffic Engineering
British Method forCapacity Calculation
The capacity is linearly dependent upon circulating flow crossing the entry.
hpcuQfFKQ cce /)(
Where: Qe = Entry capacity (pcu/h); Qc = Circulating flow across the entry (pcu/h);
Dr. Essam AlmasriAdvanced Traffic Engineering
British Method forCapacity Calculation
Geometric Factors affecting the capacity of a roundabout:
1. Inscribed circle diameter D (m)2. width of flare of each entry. E (m), 3. Approach width v (m)4. effective length over which flare is
developed l1 (m)5. entry angle ()6. entry radius (r) (m).
Dr. Essam AlmasriAdvanced Traffic Engineering
British Method forCapacity Calculation
FQfwhenQ
FQfwhenQfFKQ
cce
cccce
0
)(
Qe = Entry capacity in pcu/hQc = Circulating flow across the entry in pcu/hK = constantF = the interceptfc = the slope
Dr. Essam AlmasriAdvanced Traffic Engineering
British Method forCapacity Calculation
1 0.00347( 30) 0.978((1/ ) 0.05)K r
2
2
2
1
0.210 (1 0.2 )
303
1 0.5 /(1 exp ( 60) /10)
( ) /(1 2 )
1.6( ) /
c D
D
f t x
F x
t D
x v e v s
s e v l
Dr. Essam AlmasriAdvanced Traffic Engineering
The Ration of Flow to CapacityThe (RFC) is an indicator of the likely
performance of a junction under a future year traffic loading.
Due to site-to-site variations, there is a standard error of prediction of the entry
capacity by the formulae of 15% for any site.
Thus RFC = Flow / Capacity should be < 85%
Dr. Essam AlmasriAdvanced Traffic Engineering
Example :Find the capacity of arm A if : Flow ( Qc ) =230 veh/h Slope( fc ) = 0.8 Intercept (F) = 2371 K=0.99
( Answer: Qe=2165pcu/h )
cce QfFQ 4.111.1 For grade – separated roundabouts
If the demand flow is 1900 pcu/h. Find the RFC and comment on the results. ( Answer: RFC=0.88 88% > 85 , Not good )