4 DESIGN OF A SINGLE LANE ROUNDABOUT
4.3 Entry design
4.3.2 Splitter islands
Splitter islands are preferably used on each leg, located and shaped so as to separate and direct traffic entering and leaving the roundabout. Care must be taken, because splitter islands applied in a wrong way can cause negotiability problems for trucks and buses. They are usually kerbed, but if there is insufficient space to accommodate a kerbed island, they may consist entirely of markings and/or an overrun area.
Markings may also be used to extend a splitter island. The advantage of markings is a more flexible solution for heavy traffic.
The head of the kerbed splitter island should be set back from the side of the circulating lane by approxi-mately 1 meter to improve the roundabout’s negotiability.
Advantages of a kerbed splitter island are:
– they can act as pedestrian or cyclist refuges, provided that they are wide enough to give sufficient safe standing space for wheelchair users, pedestrians with a buggy, or a bike;
– traffic signs, public lighting and other street furniture can be sited on a kerbed splitter island;
– splitter islands increase capacity, because they allow entering drivers to distinguish earlier between vehicles exiting the roundabout and vehicles continuing around the roundabout, to which they must give way;
– they prevent wrong-way left turns onto the roundabout.
– they help keep exiting vehicles from going too fast and encroaching on the entry lanes
The shape of the kerbed splitter island of the entry lane is very critical. There are three possible designs (see figure 21):
1. radial: the kerbline lying on a straight, parallel at the axis line through the centre of the roundabout;
2. mixed radial / tangential: the kerbline lying on an arc which, when projected forward, meets the central island tangentially;
3. tangential: a straight kerbline which, when projected forward, meets the central island tangentially.
Figure 21: Design of the kerbline of the splitter island Tangential connection
Mixed radial / tangential connection
Radial connection
In the United Kingdom, France and Germany the design mentioned under number 2 is very common. The argument for this is that failure to do so can lead to more single vehicle accidents, especially on high speed roads. However, nowadays solution number 1 has been standardized in the United Kingdom and in the Netherlands only design number 1 is applied, for the following reasons:
– it keeps entry and exit speed low by maximizing deflection at the entry and exit points;
– this design supports the priority regime in which vehicles on the entry give way to circulating traffic;
– the angle between entry and circulating lane remains 90°, offering good mutual visibility.
If there is a (very) wide splitter island or refuge (> 3m, which can be beneficial to the capacity, see section 4.7), it is advisable to increase the diameter of the roundabout in order to create enough deflection on entry. Up to 30 m, the outer radius of a roundabout has little influence on speed and therefore on road safety.
In the United Kingdom (left side driving) different types of single lane roundabouts are standardized [7]
(see figure 22):
– Mini roundabout. This type of roundabout does not have a kerbed central island. There is a centrally located domed circular solid road marking up to 4 m in diameter, capable of being driven over. In the Netherlands the opinion is that the mini roundabout should only be applied where two access roads cross one other. This type of roundabout is not suitable for collector roads. This manual does not give further details on the mini roundabout.
– Compact roundabout. This type of roundabout has a kerbed central island. The compact roundabout has single lane entries and exits. The width of the circulating carriageway is such that it is not possible for two cars to pass one other.
– Normal roundabout. This type of roundabout has also a kerbed central island. The approaches may be single or dual carriageway roads. This type has flared entries and exits to allow two (or three) vehicles to enter or leave the roundabout on a given leg at the same time. If so, the circulating carriageway needs to be wide enough for two or more vehicles to travel alongside each other (lanes are not marked).
Normal roundabout (United Kingdom) Compact roundabout (United Kingdom) Figure 22: Types of roundabouts in the United Kingdom [7]
The normal roundabout in the United Kingdom arose historically from the 1960’s, when the priority rule was to give way to traffic entering the roundabout. Designs in the United Kingdom of the normal roundabout have in principal radial legs, but the practice of flaring entries tends to create tangential or mixed tangential / radial connections in France and United Kingdom. In the Netherlands roundabout entries and exits are always radial. Swedish guidelines recommend radial entries and tangential exits. This design is acceptable if pedestrian crossings are not needed.
Entry deflection is the most important factor for road safety, because it determines the speed of vehicles through the roundabout. If the normal roundabout has more than four legs, it becomes large with the probability of higher speeds on the roundabout.
Compact roundabouts can have low values of entry and exit radii in combination with rather high entry deflection. This design has less capacity than that of the normal roundabout in the United Kingdom, but:
– the road safety is safeguarded much better;
– the movements of pedestrians and cyclist can be accommodated in a much better way (see section 4.6.1).
The capacity of a compact roundabout can be improved in various ways. The Netherlands has a strong preference for the compact single lane roundabout for reason of road safety and the available possibilities for enhancing the capacity. If more capacity is needed, the creation of bypasses (see section 4.5) or a turbo roundabout (chapter 5) may be good options.
Speed control
Good roundabout design should limit entering speeds to a maximum of 30 to 35 km/h. The higher the entering speed, the higher the risk of severe accidents. With the aid of figure 23 and two formula, the expected driving speed through the roundabout can be checked. Two measurements are important: U and L. The length L (m) is the distance between the tangent of entry radius and the tangent of the exit radius.
The second measurement is U, which represents lateral deflection. The path of the vehicle is assumed to be always 1 meter away from the different curves.
Figure 23: Speed depends of deflection [3]
Vehicle path
Vehicle path
With both measurements L and U in meters, the radius of the vehicle path can be calculated.
(0,25 x L)2 + (0,50 x (U + 2))2 R vehicle path = ---
U + 2
The design is correct if the radius of the driving path is between 22 and 23 m. The relation between the speed in the vehicle path curve and the curve radius is: V = 7.4 x √R vehicle path. If the calculated speed is higher than 35 km/h, the design must be adjusted. The formula therefore is a very practical tool to determine the effect of changing design parameters.
Figure 24 shows an example from Germany of a road speed limit 100 km/h) with two roundabouts between two villages [13]. The lines indicate the strong reduction in speed caused by the roundabouts. It illustrates the effectiveness of roundabouts in reducing speeds at places with potential hazards (crossing road users).
Figure 24: Speed characteristics on a road section with two roundabouts between two villages [13]