Highway Engineering

factors supplied in Table 4.13 that are used in conjunction with present day
AADT figures to produce future flows. These are then matched with the appro-
priate AADT range in Table 4.12 in order to choose the right carriageway type.
Tr affic growth factors were issued in the COBA Manual (DoT, 1997b). This
document contained traffic growth profiles over the years 1981 to 1996 along
with forecasts of low and high traffic profiles for each year from 1997 to 2031
expressed as annual percentage growth rates.
Tr affic was split into the following five categories:

Cars (including taxis, mini-buses and camper vans)

Light goods vehicles (LGVs) – All car type delivery vans (excluding those
with twin rear tyres)

Other goods vehicles (OGV 1) – All goods vehicles with either two axles with
twin rear tyres, three axles rigid, or miscellaneous vehicles such as ambu-
lances and tractors

Other goods vehicles (OGV 2) – All goods vehicles with either three axles
articulated or four or more axles (rigid or articulated)

Buses and coaches (PSV) – Buses and mini-buses with the capacity for more
than six passengers.

Table 4.13 below gives traffic growth rates from 1997 to 2031 for the above five
categories of vehicle contained in the 1997 UK forecast. The document assumes
zero growth post 2031. Values are in annual percentage growth rates rather than
expressed as an index.
The average percentage of each vehicle type within the UK during 1994 was
as shown in Table 4.14.

Basic Elements of Highway Traffic Analysis 93

Carriageway type AADT range Quality of access

(Opening year)

Single 2-lane (2 ¥3.65 m) Up to 13 000 Restricted access, concentration of

(S2) turning movements

Wide single carriageway (2 ¥5.0 m) 6000–21 000 Restricted access, concentration of

(WS2) turning movements

Dual 2-lane all-purpose 11 000–39 000 Restricted access, concentration of

carriageways turning movements

(D2AP)

Dual 3-lane all-purpose 23 000–54 000 Severe restriction of access, left

carriageways turn only, clearway

(D3AP)

Dual 2-lane motorway Up to 41 000 Motorway restrictions apply

(D2M)

Dual 3-lane motorway 25 000–67 000 Motorway restrictions apply

(D3M)

Dual 4-lane motorway 52 000–90 000 Motorway restrictions apply

(D4M)

Table 4.12Flow ranges for different classes of rural highway