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geometric refinement mode allowing interactive testing of geometric variables. Whereas ARCADY results are for the whole time period, the output from RODEL can be specified for the whole time period or for shorter analysis periods. A new version of RODEL with crash prediction capabilities is under development.


Capacity for Urban Conditions (CETUR Formula)

The original French formula for roundabout capacities was developed in 1988 by CETUR (now known as CERTU), a government organization responsible for urban transportation guidelines nationwide (12). The CETUR formula expresses the entry capacity as a function of the impeding flow (as opposed to the circulating flow in the British and Australian methods). Similar to the U.S. method for unsignalized intersections, the impeding flow is a summation of circulating flow plus a proportion of the exiting flow at the same branch, or

Qs = Qc+Qs


Q g = i m p e d i n g f l o w , Q c = c i r c u l a t i n g f l o w , Q s = e x i t i n g f l = variable that is a function of the width of the splitter island (0.2 on average). o w ,

The theory is that the entering traffic is hampered to some degree by the exiting traffic because of the uncertainty over whether these vehicles actually exit.

Qg gets adjusted to a Qg equivalent when the circulating roadway is at least 8 m (26 ft) wide. The entry capacity C is defined as:

C = 1500 5/6 Qg for Qg < 1800 If Qg > 1800, C = 0.

With two entry lanes, entry capacity increases by 40 percent The average delay t is:

t = ( 2 0 0 0 + 2 Q g ) / ( C - Q e ) i n s e c o n d s

where Qe = entering flow.

The capacity equation is a straight line expressing the entry capacity as a function of the impeding flow. The capacity


is the maximum theoretical capacity, requiring a reserve capacity for design purposes.

Capacity for Rural Conditions (SETRA Formula)

The original capacity method for rural roundabouts was developed in 1987 by SETRA, the French national design service for rural (interurban) highways (22). This same formula is also included in the provisional SETRA guide dated January 1996 (23). It is similar to the CETUR formula, but with minor variations. Both formulas lead to linear equations relating the entry capacity to impeding traffic flows. The following SETRA formula applies to roundabouts with central islands with a radius of 15 m (49 ft) or more:

C = (1330 0.7 Qg) (1 + 0 l[le 3.5])


Q 1 l g Q li s reserve capacity percentage of reserve capacity Q e a s

= = = = = = = =

( Q c + 2 / 3 Q s ) ( 1 - 0 . 0 8 5 [ l a - 8 ] ) , ( entry width (m), width of circulatory road (m), Qs(15 - li)/15, width of splitter island, 0 when li > 15 m, C - Qe, C - Q e ) / Q e % .


Girabase is the software program developed by the regional technical study organization CETE OUEST in Nantes, France, and accepted by both the urban and interurban national design institutes (CERTU and SETRA) (49). Girabase Version 3.0 (published in March 1992) is more complex than the manual methods and takes the following parameters into consideration:

  • entry width,

  • width of circulatory roadway,

  • radius of central island,

  • width of splitter island,

  • exit width,

  • angles between consecutive branches,

  • traffic flows (vehicles or passenger car equivalent),

  • pedestrian flows, and

  • roundabout environment (urban, suburban, rural).

The empirical regression equations of Girabase are based on counts of 63,000 vehicles during 507 saturated periods of 5 to 10 minutes at 45 different roundabouts (1). The result is an exponential curve expressing the entry capacity as a function of impeding traffic. Girabase can be used for roundabouts with

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