Travel time calibration criteria are separated into two types of roadway facilities: uninterrupted flow and interrupted flow. To calibrate a model, the allowable travel time variation is measured using the equations shown in

Table 13-3

∆ = Allowable Travel Time Variation (+/- seconds)

t = Real-World Travel Time (seconds)

L = Length (ft)

S = Free Flow Speed (FFS) in mph; Posted Speed can be used for FFS if unknown

To measure and compare field data and simulation throughputs, this guidance uses the Geoffrey E. Havers (GEH) formula. The GEH statistic is typically calculated for mainline segments and ramps. It is recommended that calibration of the model using the GEH formula is calculated to a value of 3 or lower:

where,

m = output traffic throughput volumes from the simulation model (veh/h/ln)

c = traffic throughput volumes based on field data (veh/h/ln)

Additional criteria for GEH statistics are shown in

Table 13-4

and

Table 13-5

.

The FHWA 2019 Guidelines has four separate acceptability criteria. These calibration criteria are recommended for projects that involve FHWA review and approval. Each calibration criterion can be applied to all performance metrics that are being used for calibration purposes such as travel time routes, throughputs, etc.

The first two are related to sigma bands and the last two are related to accuracy. Sigma bands contain the number of outliers and inliers in the simulated results. Using graphical representation, the simulated data typically fits between the upper and lower limits of the sigma band criterion.

where,

Cr(𝑡) = Observed value of representative day (r) during time interval t

Ci(𝑡) = Observed value of non-representative day (i) within the cluster during time interval t

Csi(𝑡𝑡) = Simulated performance measure during time interval t

N

T

= Number of time intervals

N

cluster

= Number of days in the cluster representing this travel condition

Criterion III checks the simulated data with the field data to determine whether it is an overestimate or underestimate for the calibration metrics. The following equation is used to calibrate for Criterion III:

A detailed

example

of the calibration criteria calculations is provided in

Appendix N, Section 3 – Calibration Criteria Calculations Example

.

There are several differences between the FHWA 2004 and 2019 calibration guidelines. Although, calibration is still performed on similar performance measures such as travel time, throughput, queues, etc., the approach is very different when comparing the two guidance documents. Instead of each performance measure having different sets of calibration acceptance targets, all performance measures are analyzed using the same four criteria in the FHWA 2019 Guidelines. The FHWA 2019 Guidelines also has a heavy emphasis on collecting a larger amount of data for calibration.

Table 13-6

shows a comparison between the two guidance documents.