Abstract
Turbo-roundabouts with their specific design characteristics have become a suitable solution of a single-lane roundabouts capacity problem and a standard two-lane roundabouts safety problem. There is no doubt that turbo-roundabouts ensure a higher level of traffic safety in comparison to standard two-lane roundabouts due to a reduction of both conflict points and driving speed through the turbo-roundabouts. The question is if turbo-roundabouts can provide the same or a higher capacity than standard two-lane roundabouts. The answer, in the form of some important results of the basic turbo-roundabout total capacity at various traffic conditions and also in comparison with standard two-lane roundabouts, is presented in the paper. The capacity model is based on the gap-acceptance theory. Driver behaviour characteristics have been respected according to the Slovak technical guidelines. Redistribution of traffic flows into two lanes at entries and circulatory carriageway of roundabouts has been considered in the capacity calculations. An effective use of both lanes (left and right) has been assumed only at basic turbo-roundabout major entries. By contrast, low use of inner (left) lanes by drivers at entry and circulatory carriageway corresponding with real conditions in some countries has been assumed in the standard two-lane roundabout capacity calculations.
Highlights
A turbo-roundabout is a relatively new type of roundabout. It was developed by the researcher Lambertus Fortuijn from the University of Delft in the Netherlands in 1996 as a solution of a capacity problem of single-lane roundabouts and a traffic safety problem of multi-lane roundabouts
The above differences in geometry of roundabouts affect the traffic redistribution at two-lane major and minor entries, and on two-lane circulatory carriageway. Because it has a significant influence on the entry capacity, it was further considered within the capacity calculations
Since the proportion of traffic at major entries and proportion of left- and right-turning traffic have a significant influence on the intersection capacity, the following traffic conditions were modelled within capacity calculations:
Summary
A turbo-roundabout is a relatively new type of roundabout It was developed by the researcher Lambertus Fortuijn from the University of Delft in the Netherlands in 1996 as a solution of a capacity problem of single-lane roundabouts and a traffic safety problem of multi-lane roundabouts. A comparison of the total capacity of a basic turbo-roundabout and a standard two-lane roundabout (Fig. 1) at different traffic conditions is presented. The new circulating lane formed on the inner side of circulating carriageway of turbo-roundabouts (nested spiral lane) is designed opposite to the major entries (A and B, left). The physical separation of traffic lanes and the specific geometry of turbo-roundabouts are the most important features affecting traffic safety and capacity differences in comparison to two-lane roundabouts
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