Types of traffic islands; accident study method

Types of traffic islands

1. Channelising islands: triangular or teardrop islands that guide vehicles into definite paths and prevent wrong movements, usually at the junction of a minor road with a major road.
2. Dividing (separating) islands / medians: separate opposing traffic flows on the approaches of a junction or on a divided road and reduce head-on conflicts.
3. Pedestrian refuge islands: a safe place in the middle of the carriageway where pedestrians wait while crossing; at least 1.5 m wide (preferably 2.0 m) and at least as long as the crosswalk is wide.
4. Rotary (central) island: the central island of a roundabout around which vehicles circulate.
5. Splitter islands: at the entry of a rotary on each approach, to keep traffic on its own side and reduce speed.
6. Traffic control islands: for signs, signals and luminaires.
7. Loading and bus-stop islands.

Islands are made of kerbed concrete or masonry with a mountable or barrier kerb, painted with reflective markings; the nose is offset about 0.5 m from the edge of the carriageway (IRC:SP:41).

How an accident study is carried out

1. Collection of data: the police report at the site records the date, time, location, vehicles, persons, injuries, weather, road condition and cause. IRC:53 specifies standard forms (A-1 for the on-site report and A-4 for the full record). Data come from police stations, hospitals, insurance firms and road agencies; in Nepal the Traffic Police and the DoR keep the records.
2. Storing and analysis: data are coded and stored (database/GIS). Analysis includes tabulation by year, month, time of day, location, vehicle type, age and cause.
3. Condition diagram: a to-scale plan of the site showing geometry, signs and the location of the crash.
4. Collision diagram: a schematic of the crashes at the site over a period (say 3 years), with arrows showing the paths of vehicles in standard symbols; it helps to recognise patterns.
5. Spot (location) map: pins or GIS points showing crash locations; clusters identify black spots.
6. Statistical measures: crash rate per 100 million vehicle-km, per 10,000 vehicles or per 100,000 population; severity indices; comparison with the average.
7. Site investigation and cause analysis: field visit, traffic observation and conflict study; checking sight distances, friction and signs.
8. Remedial measures: engineering, education and enforcement; their cost-benefit evaluation and priority.
9. Follow-up (before/after study): monitoring to see whether the measures work.