Moving-Observer Method's Limitations In Low-Flow Roads.pdf

The moving observer method for estimating traffic flow rates involves an observer in a moving vehicle counting oncoming vehicles passed in a specified length of road. The average speed of the moving vehicle, length of road, and trip time are used to calculate the flow rate. This study investigated the feasibility and accuracy of using this method for rural roads in New Zealand with low flow rates. A risk analysis approach was used to simulate moving observer surveys on roads with low flow rates. Results show that this method does not give accurate flow estimates for roads with a balanced two-way flow rate less than about 1000 veh/h, unless an impractically large number of moving observer runs are undertaken.

The moving observer method has several advantages, including the ability to collect information on flow and speed simultaneously, measure travel time, and classify vehicles. However, it is sensitive to intersecting traffic from side streets and requires a large number of runs to achieve accurate results.

The average annual daily traffic volume (AADT) is a critical parameter in roading project evaluations, and reducing the error in its estimation is beneficial in reducing overall analysis uncertainty. Traffic flow rates can be measured using stationary counts, but this method has disadvantages, including the requirement for specialist equipment and limited coverage.

The moving observer method was developed in the UK by the Road Research Laboratory and was first described in a paper by Wardrop and Charlesworth (1954). The method involves a series of runs in a test vehicle made travelling 'with' and 'against' a one-way traffic stream. The observers in the test vehicles record the number of opposing vehicles met, the number of vehicles overtaking the test vehicle, the number of vehicles the test vehicle overtook, and the average speed of the test vehicle and the distance of the run.

The moving observer method has several disadvantages, including sensitivity to intersecting traffic from side streets and the requirement for a large number of runs to achieve accurate results. However, it is economical and can provide additional information such as the locations and causes of delays.

The risk analysis process involves developing a model, identifying uncertainty in variables, assigning a probability distribution, and analysing the model by simulation to determine the range and probability of all outcomes. The @RISK software is used to simulate the moving observer method and estimate the uncertainty in the flow rate estimates.

The study used a risk analysis approach to simulate moving observer surveys on roads with low flow rates. The results show that the moving observer method does not give accurate flow estimates for roads with a balanced two-way flow rate less than about 1000 veh/h, unless an impractically large number of moving observer runs are undertaken. The study also highlights the importance of reducing the error in the estimation of the traffic volume to reduce overall analysis uncertainty.