Sustained Flow Index: Stochastic Measure of Freeway Performance

Abstract

Research into the operation of traffic flow at high volumes reveals that the capacity of freeways is not a fixed number but is rather a random variable. Since traditional operational performance measures for the analysis of traffic flow on freeways typically disregard the randomness of capacity, new approaches to make use of the concept of randomness for freeway operation analysis are required. To address that need, this paper introduces a new indicator of freeway performance based solely on a stochastic approach to capacity computation. With this new indicator, the maximum reliable volume that can be carried by a freeway over prolonged time periods was derived from parameters of capacity distribution functions. The breakdown probability corresponding to the optimum volume can be used to select a single value from the capacity distribution function. To explore the empirical relationship between expected values of capacity and the optimum volumes calculated from this new computational process, flow data from several German freeway sections were analyzed. To illustrate the application potential for a sustained flow index, this paper also discusses the bases for increasing the effectiveness of ramp metering and vehicle routing management techniques.

Estimation of the Effect of Rain and Incidents on Freeway Capacity and Free-Flow Speed

“…The definition of capacity has been studied extensively in the literature (14)(15)(16)(17). Generally, as flow increases, the speed gradually decreases until the breakdown phenomenon occurs.…”

Lane‐changing and freeway capacity: A Bayesian inference stochastic model

“…So, in a particular application, any desired probability level could be chosen in the tuning of the control algorithm. This concept is similar to the Sustained Flow Index (SFI) that addresses the trade‐off between flow and reliability (Shojaat, Geistefeldt, Parr, Wilmot, & Wolshon, ).…”

Optimum Volume of Freeway Corridors

“…Based on the concept of stochastic capacities, Brilon defined traffic efficiency as a measure that can be maximized to derive a traffic volume at which the optimum performance of a freeway facility is achieved ( 14 ). More recently, Shojaat et al introduced the SFI as a joint performance measure for freeways that reflects the tradeoff between maximizing the traffic throughput and minimizing the risk of a traffic breakdown ( 3 ). The volume that maximizes the SFI, referred to as the optimum volume, was analytically derived as a function of the parameters of the capacity distribution function.…”

“…On the other hand, underutilization of a freeway should be avoided. Representing the tradeoff between sustaining fluid traffic conditions and maximizing the traffic throughput, the sustained flow index (SFI) was recently introduced as a joint performance measure ( 3 ). The SFI is defined as the product of the traffic volume and the probability of survival (i.e., complement of the breakdown probability).…”