5.2.4 Procedure for Determining Design Traffic
Single carriageway – two-way traffic
A single carriageway is the most common sealed road pavement in rural areas and traffic needs only to be apportioned to each lane. The width of sealed pavement influences the traffic pattern. Assuming that traffic is equal in both directions, Table 5.1 provides a guide to estimating the design traffic.
Table 5.1: Estimation of design traffic from AADT for single carriageways
| Width of seal | Estimated design traffic (v/l/d) | Comment | |
|---|---|---|---|
| 3.7–5.6 m | AADT | Seal width is considered too narrow for 2 lanes | |
| > 5.6 m | ½ × AADT | Traffic is considered to predominantly travel in distinct lanes on seals of this width, especially if the centre line and/or lanes are line marked | |
| Sealed shoulders, parking lanes, identified by edge line marking to be separate from the traffic lanes | Adopt < 100 (minimum voids factor) | If not line marked, or on winding alignments, some of the traffic may wander onto the shoulder and < 100 v/l/d may not be appropriate. If in doubt, a traffic count should be conducted, or adopt the design traffic volume from the adjacent lane | |
| Overtaking lanes (in one direction) | 60–80% of ½ × AADT | Determine % HV for each lane as a proportion of the total traffic volume in that lane | If in doubt, arrange a traffic count for each lane |
| Left-hand lane (3.7 m) | |||
| Right-hand lane (3.7 m) | 20–40% of ½ × AADT | ||
| Single lane in opposite direction | ½ × AADT | %HV same as in AADT | |
| On and off ramps on freeways or urban road systems | Traffic volumes (AADT) before and past the ramp may provide a good indication of AADT on the ramp. Otherwise, arrange a traffic count. Traffic volume on the road connected to the ramp may also provide additional useful information to determine AADT on the ramp | ||
| Service roads to major roads | For one-way traffic, the design traffic is equal to the AADT For two-way traffic, use ½ AADT | AADT refers to traffic using the service road only. If not available, arrange a traffic count | |
Dual carriageway – one-way traffic
AADT is usually defined as the total traffic carried by both carriageways, but this should be confirmed. Where this is the case, the first step is to determine the traffic on each carriageway, and this is generally assumed to be ½ AADT.
For heavily trafficked roads, with more than two lanes in each direction, an actual traffic count may be available for each lane and this should be the traffic volume used in the design.
For rural freeways and highways, or duplicated roads into rural townships (classed as urban‑type locations) with medium to high traffic volumes, Table 5.2 provides a guide to estimating the design traffic from AADT.
Generally, the heavy vehicles travel in the left-hand lanes on multi-lane carriageways, occasionally using the climbing/passing lanes provided that the additional lane is of sufficient length to allow the heavy vehicles to change lanes without undue interference.
Table 5.2: Estimation of design traffic from AADT for dual carriageways
| Lane (assumed 3.7 m wide) | Estimated design traffic (v/l/d) | Comments | |
|---|---|---|---|
| Multi-lane, heavily trafficked | ½ AADT divided by the number of lanes in the carriageway | These roads are usually in urban areas or linking major centres. Traffic volume is often > 2000 v/l/d in all lanes but the % heavy vehicles may vary between lanes | |
| Two- lane carriageway: | 60 to 80% of ½ AADT | Generally 60% for urban/80% for rural | Each carriageway = ½ AADT |
| left-hand (outer) lane | |||
| right-hand (inner) lane | 40 to 20% of ½ AADT | Generally 40% for urban/ 20% for rural | |
| Sealed shoulders, parking lanes identified by edge line marking to be separate from the traffic lanes | Adopt < 100 | On some busy roads, trucks may tend to travel partially on the shoulder, and this must be taken into account. A traffic count should be conducted, and/or traffic pattern determined | |
| Where two lanes merge into one (at end of a duplicated section) | ½ AADT | Merged traffic is ½ AADT, but design of binder application rates and layout of sprayer runs within the merge area require particular care | |
| Overtaking lanes | |||
| Off and on ramps | % of ½ x AADT | If actual traffic counts are not available for ramps, traffic on the side road, before and past the ramp, may provide an indication of the traffic volume using the ramp | |