10.2.2 Entry Ramp Alignment at Minor Road Terminals
The right-turn roadway
An example of an entry ramp terminal is shown in Figure 10.2. The location of the turning roadway is fixed by the vertical alignment of the minor road, the horizontal alignment of the ramp and the design vehicle turning template (external radius typically in the range of 20 to 30 m). The swept path of the template should clear the kerbs by 0.5 m on both sides.
To facilitate recognition of the entry ramp on spread diamonds, the width of the throat based on turning templates should be increased by a splay of 1.5 m on the inside of the curve and 1 m on the outside of the curve. On closed diamonds, the offset on the inside may have to be reduced to 1 m or 0.5 m depending on the space available (Figure 10.4).
Left-turn into entry ramp
Desirably a left-turn lane into an entry ramp should be formed from a single left-hand curve or a curve followed by a straight, assisted by positive superelevation. Reverse curves should only be used where approach speeds are low. The radius of the left turn is related to the angle of the ramp to the minor road θ (refer to Figure 10.2). In rural areas it may be desirable to allow a relatively high-speed left turn onto the ramp. However, in urban areas where pedestrians cross at the terminal the operating speed of the left turn should be relatively low (e.g. 20 to 30 km/h).