Requiem for Freeway Travel Time Estimation Methods Based on Blind Speed Interpolations Between Point Measurements

Travel time estimation from loop measurements has attracted extensive research in the last decade, resulting in numerous methodologies. Among these, those that rely on spot speed measurements at detector sites to obtain travel time estimation on the target stretch are the most intuitive. The key issue concerning these methods is the spatial generalization of point measurements over a freeway link. This paper shows that all speed interpolation methods that omit traffic dynamics and queue evolution do not contribute to better travel time estimations. All methods are inaccurate in congested and transition conditions, and the claimed relative benefits using various speed interpolation methods result from context-specific experiments. Therefore, these methods should be carefully used and not taken as perfect. Lacking a better approach, it is recommended to avoid overcomplicated mathematical interpolations and focus efforts on intelligent smoothing of the noisy loop detector data, reducing the fluctuations of short time interval aggregations while maintaining the immediacy of the measurements.