Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference - May 19, 2009 Presented by: Jaesup Lee, Virginia Department of Transportation Dean Munn, The Corradino Group

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Introduction Traffic Data used in basic analysis and model estimation Data Development and Definition –Free Flow Speed –Traffic Flow (Uninterrupted vs. Interrupted) –Link Capacity Various Curve Fittings by Functional Class Findings and Further Research Outline

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Introduction Project Goals Use empirical data obtained from Virginia facilities to evaluate speed-flow relationships Test various volume-delay functional forms for each facility type and determine which provides the best performance Calibrate volume delay function parameters for each facility type Outcome should be suitable for implementation in Virginia urban travel models

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Introduction Fundamental Elements of Volume-Delay Estimation Converting spot speeds to space-mean speed Characteristics of free-flow Identifying boundary between uninterrupted and interrupted flow Using knowledge of this boundary to estimate the maximum sustainable flow rate (Capacity) Use empirical observations to fit VDF curve parameters

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Traffic Monitoring System (TSM) Data 5,848 locations from 17,400 detector locations available Three locations per classification selected

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Traffic Data Data records are a summary of each 15 minute period Speed bins are in 5 mph increments Data records are organized by lane and vehicle class CUBE/Voyager script simplifies data Spot speeds are converted to Space Mean Speeds

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Processed Data Speed vs Density Speed vs Flow Speed by Time of Day

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Estimating Free Flow Speed 1.HCM recommends using mean value for low volume conditions 2.Standard practice also includes using 85 th Percentile speed 71.1 mph 73.7 mph

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Defining Interrupted Flow 1.Plots of flow vs density and speed vs density show two flow states 2.Others have defined the transition point as the maximum flow or the density at maximum speed, but this is not representative of typical conditions 3.Statistical techniques can define the transition between the two states Flow vs. Density Speed vs. Density Interrupted Flow

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Defining Interrupted Flow We define interrupted flow as: Any speed below the threshold where there is % probability that it is the same as freeflow. For our Rural Freeway example: The computed threshold is MPH

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Defining Interrupted Flow Rural Freeway example, with flow states identified

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Interrupted Flow Histogram – Percent with Interrupted Flow vs. Flow Density Percent with Interrupted Flow

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Estimating Capacity 1.Our data shows a classic logistic distribution 2.We estimated parameters (using density as the only variable) to create a probability function that best fits the data 3.Capacity corresponds to flow density with a 50% probability of being interrupted P I = 1/[1 + e (  1 D+  0 ) ], where D = Density (veh/mi)

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, % Probability of Interrupted Flow 0.50% Probability of Interrupted Flow 50.0% Probability of Interrupted Flow Capacity Estimates This example gives a 39.7 pc/mi Density Threshold or a 2384 pc/hr Max Flow Rate

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Fitting Volume Delay Functions Volume-Delay Functions - Using the computed capacity, the following volume delay functions were estimated based on speeds during uninterrupted flow BPR:R = R 0 [1 +  (V/C)^  ] Akcelik: R=R 0 +D T[(V/C-1)+sqrt{(V/C-1) 2 +(16J(V/C)L 2 )/T 2 }] Conical: R=R 0 [2 + sqrt(  2 (1-V/C) +  2 ) –  (1-V/C) –  ]

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Curve Fitting - non-linear regression Goodness of Fit –R-squared –Root Mean Square Error –Non-Parametric tests e.g. Chi-Square Other Criteria - suitability for model applications Fitting Volume Delay Functions

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Fitting Volume Delay Functions Urban Interstate

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Fitting Volume Delay Functions Rural Interstate

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Fitting Volume Delay Functions Urban Expressway

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Fitting Volume Delay Functions Rural Principal Arterial

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Fitting Volume Delay Functions Urban Other Principal Arterials

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Fitting Volume Delay Functions Rural Minor Arterial

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Fitting Volume Delay Functions Urban Minor Arterial

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Fitting Volume Delay Functions Rural Collector

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Fitting Volume Delay Functions Urban Collector

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Fitting Volume Delay Functions Rural Local

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Fitting Volume Delay Functions Summary of calibrated inputs to VDF fitting process

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Fitting Volume Delay Functions Summarized results from VDF fitting process

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Initial Findings Standard VDF functions are all capable of performing adequately across road classes For a given road class, VDF parameters fitted for one location, seem to be transferable to other locations Goodness of fit measures do not strongly differentiate between functions The Akcelik function, with its more rigorous theoretical underpinnings, seems to work very well

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Next Steps Additional facility types Check model transferability to other facilities Compare HCM capacity, planning capacity, and empirical capacity Continue to automate analysis process Test functions in urban models (assignment convergence, average travel speeds) New VDF functional forms and calibrated parameters will become part VDOT modeling standards

Investigation of Speed-Flow Relations and Estimation of Volume Delay Functions for Travel Demand Models in Virginia TRB Planning Applications Conference, May 19, 2009 Q & A Thank you ! Contact points Jaesup Lee: Dean Munn: Jeremy Raw :