5.2.1 Number of Lanes on Ramps
Ramps should have the number of lanes required to provide a satisfactory level of service on the design element and to avoid traffic queues from adversely affecting adjacent elements. This requirement applies to the approaches to ramp terminals and generally throughout the length of the ramp. Direct or semi-direct ramps may have one lane but are often designed for two-lane operation with provision for emergency parking, unless the expected volume exceeds the capacity of two through lanes. In this case, three-lane operation on the ramp should be considered.
Exit and entry ramps may be one lane or two lanes at the nose depending on traffic volumes. Where two lanes are required at the nose to meet the traffic demand, the cross-section should allow for a stalled vehicle to be passed in addition to the full traffic lanes.
Where the ramp caters for a relatively low volume of traffic a single lane at the exit nose and along the ramp will suffice as shown in Figure 11.1. The shoulder of the freeway is carried past the nose and onto the ramp to provide for emergency stopping or vehicle breakdown.
Single-lane (at the nose) exit ramps should be widened to two lanes on the ramp as shown in Figure 11.2 when:
- a truck will exit at less than 50 km/h at the nose, and a significant number of trucks use the ramp
- the ramp is longer than 600 m.
The transition from two lanes to one lane at the nose should be implemented as shown in Figure 11.2.
Additional lanes are often required at exit ramp terminals with the minor road to provide adequate intersection capacity. Relatively short exit ramps may be widened over their entire length to cater for storage requirements at the minor road intersection (e.g. in urban situations).
Two-lane exit ramps with a single lane at the nose are effectively one-lane ramps with provision for overtaking. It is therefore not necessary to have full shoulder widths on these ramps. A 1.0 m shoulder on each side to support the pavement is sufficient.
Where an entry ramp caters for a relatively low volume of traffic a single lane at the nose and a single lane along the ramp are often adequate, a shoulder being provided past the nose and on the ramp (Figure 11.6 and Figure 11.7(a)).
For an entry ramp with a single lane at the nose, and with a design speed of the through roadway of 80 km/h or more, a second entry ramp lane should be provided when:
- the length of a single-lane ramp exceeds 300 m on a level grade and a truck accelerating from rest at the ramp terminus would not be expected to reach 50 km/h at the nose, and a significant number of trucks use the ramp
- a very long (i.e. length > 600 m) entry ramp is provided.
If the design year traffic volumes require two lanes on an entry ramp, two alternative approaches may be taken, namely provide a:
- single lane at the nose, using the restricted capacity of this lane to control the volume of traffic entering the freeway (Figure 11.7(c))
- full two-lane entry with an added lane on the freeway. These ramps require the capacity of the two lanes and should therefore be provided with sufficient shoulder width to allow a stalled vehicle to be passed (Figure 11.7(b)).