Vehicle Occupancy Survey

There are three most recognized methods of occupancy surveys that currently exist; roadside/windshield method, carousel method, video surveillance method. However, the roadside/windshield method is the most generally recommended over other methods. Orange To Green Solutions, therefore, use the windshield method. This method involves stationing one or more observers along the roadside to count vehicles and occupants. Record keeping may be by paper and pencil, by an electronic counter or by a computer. For safety purposes, the observers must stand in a protected zone such as behind the guardrail, or on an overpass, or within a reasonable distance from the traffic flow. This may limit the observer’s view.

In implementing the windshield data collection method, the planning process has to consider conditions that can bias the results. The following groups of conditions have to be considered.

  • Traffic Characteristics – The type of locations, spacing, speed of traffic, traffic density, traffic weave, average occupancy, and type of vehicle.
  • Temporal Conditions – Normal working day.
  • Environmental Conditions – The Appropriate weather conditions.
  • Human Factors – Observer fatigue and observation distance.

Field Operations – After a region, corridor, or site has been selected, several observation sites and survey time periods are randomly chosen for collecting data. At each observation site, one or more observers are stationed to monitor the lanes of traffic. Each observer is assigned one or more distinct lanes to monitor, depending on the traffic volume. Observers are instructed to collect data only on certain vehicle types. Usually, the targeted vehicles are passenger cars, pick-ups, vans, and motorcycles. For each targeted vehicles passing the observer, the observer records the number of occupants within the vehicle and vehicle type.
Given the quantity of data that is collected with this method, it is more important for observers to record occupancy data for vehicles that they clearly see rather than a recording a value for every vehicle. Selective sampling of the traffic, such as collecting data on every other vehicle or for certain time periods, could help improve the observer’s accuracy while obtaining a representative sample of the traffic.

Data Processing – Data will be tabulated in an excel sheet, whether it is collected in an electronic tablet or on a survey form.

Sample Collection Response – For low volume traffic, 90% or more of the vehicles can be captured from a single lane. For high volume traffic, 75% or more of the vehicles can be captured from a single lane. For a multi-lane road segment, all lanes are typically observed (usually requiring more than one observer). Collecting data for 30 minutes with a 10 minutes break may reduce observers’ fatigue and improve the accuracy of the data.

Strength – Requires little equipment, Easy to implement, and Standard method, which produces results comparable to historical data.