01557646

Component

Travel time reliability (TTR) is becoming a significant performance aspect of the transportation system, both for passenger and freight movements. It is defined by how travel times for a facility or trip vary over time, due to the variable occurrence of disruptions and fluctuating demand. Much research has gone into developing metrics for measuring TTR performance – which are based on the distribution of travel times – and include both statistical measures and “on-time” or “failure” measures. Many agencies have begun reporting TTR performance using these measures and TTR performance measures are being considered for inclusion into the Highway Capacity Manual (HCM). However, relating TTR performance measures to quality of service has been lacking. This paper deals with this issue by defining a service measure for TTR for freeway facilities using the level of service (LOS) classification concept in the HCM, which is based on six levels (“A” through “F”) that define quality of service. Several TTR performance measures are explored for the service measure as well as different ways to define the LOS classification scheme. Because TTR is defined by the distribution of travel times, the definition of LOS is considerably more complex than those based on a mean or typical value. After testing the various measures and classification schemes with continuously collected data from freeways in multiple urbanized areas throughout the nation, the recommended TTR service measure is the fifth percentile travel speed and LOS thresholds are provided. Many other measures could be used from a technical standpoint, but travel speed is thought to be more easily understood by nontechnical audiences and is consistent with performance measures already defined in the HCM. It is recommended that travel time reliability be the service measure for freeway facilities which are part of an extensive freeway network in large urbanized areas, those over 500.000 population. An extensive freeway network is defined as having at least three freeway facilities. Analysts should use their judgment in cases that do not meet these. Density would continue to be used on all other freeway facilities as well as for other aspects of freeways covered by the HCM (e.g., freeway segments). At a minimum, the TTR LOS defined here will provide guidance to analysts in determining acceptable levels of performance. The Florida Department of Transportation (FDOT) plans to use the TTR LOS as a way to identify mobility performance deficiencies on Florida freeways in major urbanized areas. The usefulness of the TTR LOS will be expanded if it is included in the HCM as the service measure for TTR, recognizing that the HCM procedures will still produce multiple performance measures for TTR. It also is expected that the TTR LOS will be an aid in setting transportation system and freight performance targets.

This paper was sponsored by TRB committee AHB40 Highway Capacity and Quality of Service.

01550057

15-4381

English

20p

2015

Transportation Research Board 94th Annual Meeting

Digital/other

Figures; Tables

Freeways; Level of service; Performance measurement; Quality of service; Reliability; Travel time; Urban areas

Florida Department of Transportation; Highway Capacity Manual

Highways; Planning and Forecasting; I72: Traffic and Transport Planning

Transportation Research Board Annual Meeting 2015 Paper #15-4381

TRIS, TRB, ATRI

Dec 30 2014 1:26PM