Motorway owners and operators are now using improvements in travel time reliability (a reduction in travel time variability) as an added benefit of motorway use and a key performance indicator (KPI).This paper introduces a novel use of the Fundamental Diagram (FD) to monitor travel times and performance on a motorway. The methodology is tested in a case study in collaboration with Leighton Contractor’s on a new motorway. KPIs on travel times are to be used to financially incentivise the operator to maintain a high level of travel time reliability. The KPI definitions that were initially proposed were based on criteria that would have led to penalties being applied even under normal operation. Hence a more robust and equitable approach to KPI calculation was required.
We report on the design and development of such an approach, which has been accepted by all stakeholders in the project. It is based on the Fundamental Diagram, which is used to define a base line for equilibrium traffic conditions. Calculation of the Fundamental Diagram is based on a recently proposed model. We then construct an envelope around the FD. The boundaries of the envelope signify normal or “healthy” traffic conditions. Travel time KPIs penalties are only triggered for traffic flow data points that are outside the agreed “healthy” flow envelope.