For a reliable spot speed study, what is the generally considered sample size of vehicles per lane?

For a reliable spot speed study, the generally accepted sample size of vehicles per lane is 100 vehicles. This sample size is considered sufficient to capture variability in speed data while providing a statistically robust representation of traffic conditions.

Using 100 vehicles allows for a good balance between data reliability and the practicalities of conducting the study. It is large enough to smooth out random fluctuations in traffic speed that could occur due to variations in driver behavior, environmental conditions, or traffic density. Additionally, this size helps in ensuring that the data collected can be generalized to other similar conditions without being overly influenced by outliers or anomalies.

Choosing a sample size smaller than 100 could lead to a lack of statistical power and insufficient representation of the traffic stream, which may result in unreliable conclusions. Conversely, while larger sample sizes, such as 200 or 500 vehicles, could yield very accurate estimates, they may not be necessary for most studies and could require more time and resources, potentially making them impractical for routine applications.