General Methodology

Two types of traffic counts are conducted: annual counts and short-term counts.
By comparing short-term counts taken during a defined period with annual count results from roads with similar seasonal traffic patterns, it is possible to estimate Annual Average Daily Traffic (AADT), Summer Daily Traffic (SDT) and Winter Daily Traffic (WDT).

For locations where traffic is measured throughout the entire year, three types of equipment are used:

1. Traffic counters with full communication that are regularly accessed remotely.
   These devices record not only traffic volume but also, for example, vehicle speeds.

2. Traffic counters with limited communication, accessed remotely for traffic volume only.
   Many of these are connected to weather stations.

3. Non-communicating counters, which must be visited in the field to retrieve data.

There are currently around 300 annual count sites.
Each counting section may contain one or multiple counters.
Each year, short-term counts are carried out at approximately 200 additional sites.

Where continuous counting is not performed, traffic volumes are extrapolated between years using adjustment factors derived from nearby annual count stations.

Through this extensive traffic monitoring program, the Icelandic Road and Coastal Administration has collected valuable information on the long-term development of traffic and vehicle-kilometres travelled on national roads.
These data are essential for many of the agency’s planning, safety and operational activities.

Methodology for Permanent Traffic Counts

General methodology and averaging methods for permanent counters

A permanent count site is a location where traffic is recorded continuously every day of the year.
Traffic counts may be recorded for each individual lane or as a combined total for both directions on two-lane roads, depending on the equipment installed at each site.

In the formulas below:
• u = daily traffic volume (vehicles per 24 hours)
• ∑ = summation
• n = number of days (without subscript: total days in the year)
• s = subscript denoting summer months (see abbreviations and definitions above)
• v = subscript denoting winter months (see abbreviations and definitions above)

Based on these definitions, the average traffic indicators for permanent count stations are calculated as follows:

Formulas

AADT — Annual Average Daily Traffic

AADT=∑unAADT = \frac{\sum u}{n}AADT=n∑u​

SDT — Summer Daily Traffic

SDT=∑usnsSDT = \frac{\sum u_s}{n_s}SDT=ns​∑us​​

WDT — Winter Daily Traffic

WDT=∑uvnvWDT = \frac{\sum u_v}{n_v}WDT=nv​∑uv​​

Methodology for Short-Term Traffic Counts

General methodology for calculating averages from short-term counts

Iceland is divided into seven traffic counting regions:
South Iceland, Reykjanes, West Iceland, Westfjords, North Iceland, East Iceland and the Highlands.

Short-term counts are conducted each year in at least one of these regions.
Counts are performed on primary, secondary and regional roads, but generally not planned on local roads.

Counting takes place during the period May–September.
Counters are placed at locations that best represent average traffic conditions for the relevant road section or sub-section (stallur).

All equipment is configured to record one daily traffic value per 24 hours.
The counting period is typically at least 3 weeks in duration.

Calculation notes and definitions

In the formulas below:
• u = daily traffic volume at the counting site
• st = index referring to the short-term count site
• vm = index referring to the reference permanent count site
• ∑ = summation
 e.g., ∑u_st = total traffic at the short-term site during the counting period
   ∑u_vm = total traffic at the reference site during the same period

A reference site is usually the nearest permanent count station within the same climatic and traffic distribution area, considered to have comparable seasonal patterns.

Based on these definitions, the formulas for estimating averages from short-term counts are derived accordingly:

AADT (Annual Average Daily Traffic):

AADT = (Sum of all daily traffic counts in the year) / (Number of days in the year)

AADT = ∑u / n

SDT (Summer Daily Traffic):

SDT = (Sum of daily traffic counts during summer) / (Number of summer days)

SDT = ∑uₛ / nₛ

WDT (Winter Daily Traffic):

WDT = (Sum of daily traffic counts during winter) / (Number of winter days)

WDT = ∑uᵥ / nᵥ

Important considerations

• Short-term counts are not intended to measure traffic peaks or minimum volumes.
• The accuracy of calculated AADT, SDT and WDT depends on traffic characteristics and flow levels.
 → Based on experience, accuracy averages around 90%.
• Accuracy is typically highest in urban and suburban areas and lower on low-volume summer roads.
• Processed count data and derived averages are published in Q1 of the following calendar year.
• Equipment failure rates for short-term counters are typically 5–10%.

Annual Publication of Traffic Data on National Roads

Each year, the Icelandic Road and Coastal Administration publishes traffic data for almost the entire national road network
(traffic is generally not calculated for local roads).
These traffic figures are presented as average traffic values (AADT, SDT and WDT) for defined sections of each road — either a road section or a sub-section (stallur).

Traffic averages are calculated using both permanent and short-term counts:

• A permanent count site is a location where traffic is recorded continuously every day of the year.
• A short-term count is typically carried out between May and September and lasts 3 to 6 weeks on selected roads.

Published averages represent two-way traffic (traffic in both directions combined).

Since only a small portion of road sections have permanent counting equipment, traffic for the majority of sections is estimated using data from the nearest permanent count site with similar traffic distribution.
Short-term counts are generally repeated every 4–7 years on these road sections, which may cause calculated averages to not always follow year-to-year changes at the reference site.

Definitions

Road section

A defined part of a road.
For example, the Ring Road has road number 1, but its sections are labelled 1-a1 to 1-z2, each with a defined length.

Stallur (sub-section)

A smaller division within a road section.
A section such as 1-a4 may be divided into multiple sub-sections (stallar),
e.g.:
• Stallur 1: station 0 – 6,280 m
• Stallur 2: station 6,280 – 11,410 m

Road sections and sub-sections have defined start and end chainages
(the distance difference between those two points equals the section length).

Traffic Direction

All published averages (AADT, SDT and WDT) represent combined traffic in both directions.

Detailed Traffic Data

On high-traffic roads, information on hourly traffic volumes and 15-minute traffic volumes is sometimes required.
Such data are available at all locations equipped with traffic counters that support remote communication, provided they are not connected to weather stations.
At locations with weather-station counters, 10-minute traffic data are available.

Older types of traffic counters only record one value per 24 hours and must be manually reconfigured to record more frequently.

Movement (Turning) Counts at Intersections

For the calibration and timing of traffic signals at intersections, 15-minute turning counts are required.
These are obtained through a method called turning-movement counts, where staff manually record traffic entering the intersection on all approaches at 15-minute intervals:

• for 2–3 hours in the morning
• and 3–4 hours in the afternoon
• typically on a Friday