Research Trends and Results

Preparation of the Time Reliability Index Value
Calculation Manual
MOROTA Keiji, Researcher
SEKIYA Hirotaka, Senior Researcher
TAKAMIYA Susumu, Head (Dr. Eng.)
Traffic Engineering Division, Road Department(Key words) Road traffic, time reliability, probe data
1. Introduction
According to the time a motorway opens for service,
travel time on sections of parallel ordinary roads may
shorten. And when this happens, not only average
travel time, but the 95-percentile travel time (in asense, travel time during congestion which occurs
once every 20 days) for example, is also shortened(Fig. 1). In these cases, the confidence in travel timeon the road felt by road users who set their departuretime predicting the 95 percentile travel time will beincreased, because they will be able to set a later
departure time than the departure time before the new
motorway opened for service.
The Traffic Engineering Division has studied amethod of using probe travel time data to calculate
index values which indicate such time reliability, andhas summarized the findings in the Time Reliability
Index Value Calculation Manual (below called “themanual”). We presume that regional development
bureaus will use this manual to calculate time
reliability index values in order to measure theeffectiveness of a road project.
2. Calculation of the time reliability index value inthe manual
Figure 2 is a flow chart of the time reliability index
value calculation method presented in the manual. A
characteristic of the manual is that it presents themethods: [4. Preparing OD section data], and 5.
[Setting the reliability ranking of time reliability index
values].
[4. Preparing OD section data] presents a method ofpreparing travel times for sections consisting of anumber of connected DRM (Digital Road Map)
sections (below called, “OD Sections”) based on
section unit probe travel time data of DRM.
Compensation is done in cases where, during theprocessing of preparing OD section travel times, a
DRM section for which probe travel time data wasunavailable (below called, “missed measurementsection”) is found. Compensation for a missing
measurement section is done using average travel timefor the DRM section.
[5. Setting the reliability ranking of time reliability

index values] presents a method of assessingreliability: a factor indicating the certainty ofcalculated index values. Reliability has been definedas probability that the difference between the index
value calculated based on data for a number of days
which can actually be obtained and the actual value(calculated value when there are no missed
measurements, and data for all days could beobtained) is within ±5% of the actual value. Reliability
is set as four ranks from A to D based on the numberof days when data could be obtained, and is ayardstick indicating the probability of the calculated
index value. And when the reliability rank does notsatisfy the ranking which should be ensured, as
necessary, the state of missed measurement sections
can be checked to set sections within the OD sections
whose index value is calculated.
3. Conclusions
The manual will be distributed to regional
development bureaus, where it is counted on to be
used not only to measure the effectiveness of roadprojects, but also to specify and prioritize sections
where countermeasures are required.
Figure 1. Change of Required time on National Highway
No. 1 (near Amanokawa Intersection: about 3km) after
Opening of the Daini Keihan Expressway (2010)

Figure 2. Time Reliability Index Value Calculation FlowChart