Table of Contents

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 sealEstimated design traffic
3.7–5.6 mAADTSeal width is considered too narrow for 2 lanes
> 5.6 m½ × AADTTraffic 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 lanesAdopt < 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 × AADTDetermine % HV for each lane as a proportion of the total traffic volume in that laneIf 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 direction½ × AADT%HV same as in AADT
On and off ramps on freeways or urban road systemsTraffic 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

(assumed 3.7 m wide)
Estimated design traffic
Multi-lane, heavily trafficked

½ AADT divided by the number of lanes in the carriageway
½ AADT x % traffic in each lane

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 AADTGenerally 60% for urban/80% for ruralEach carriageway = ½ AADT
left-hand (outer) lane
right-hand (inner) lane40 to 20% of ½ AADTGenerally 40% for urban/ 20% for rural
Sealed shoulders, parking lanes identified by edge line marking to be separate from the traffic lanesAdopt < 100On 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 AADTMerged 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 AADTIf 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