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30 The minimum number of observations required to estab- public and to decrease response time to incidents for safety lish a target confidence interval for the mean travel time or and congestion relief reasons. Most TMCs gather real-time the mean delay is given by the following equation: traffic data using stationary devices such as in-pavement in- 2 duction loop detectors, closed circuit television cameras, and s other mounted systems. The future of data collection may in- N = 4 * t (1- /2),N -1 * (Eq. 3.1) CI1- % clude gathering moving vehicle information from mobile phones and in-vehicle Global Positioning Systems (GPS) where components. On freeways, in-pavement induction loop N = Minimum required number of observations; detectors are most common and collect traffic flow (vehicles CI(1-alpha)% = The confidence interval for the true mean per hour per lane), instantaneous speeds at the detector, and with probability of (1-alpha)%, where "alpha" detector occupancy (fraction per time interval that vehicles equals the probability of the true mean not occupy the detector). If the detectors in a roadway segment lying within the confidence interval; or facility are mostly functional and are located close together t(1-alpha/2), N-1 = The Student's t statistic for the probability of (no more than one mile apart), reasonable travel-time esti- two-sided error summing to alpha with N - 1 mates can be made from the instantaneous speed data. degrees of freedom; and A few agencies store this data and make it available to per- s = The standard deviation in the measured travel sons outside of the agency. Some TMCs, however, do not store times and the square root of the variance. their real-time data for more than 24 hours and do not make the stored data accessible to persons outside of the agency. Exhibit 3.5 illustrates the minimum number of observations Storage and dissemination of traffic data are technically feasi- required for various target levels of precision, expressed here in ble. What is often the barrier is the lack of appointed responsi- units of the standard deviation of the measured travel times or bilities within the agency for data archiving, lack of a use for the delay times. The desired precision is defined as the desired con- data beyond the operation of the roadway, and developing fidence interval (CI) in seconds divided by standard deviation policies for public access to the data. TMCs that collect real- (S) in seconds. For example, if the standard deviation in the time traffic data are primarily concerned with real-time oper- delay is 1.5 seconds and the desired confidence interval is 3.0 ations and are not funded or given directives for archiving data seconds, the desired precision is 2.0 (i.e., 3.0 divided by 1.5 for nonagency use. Nonetheless, as transportation analysis equals 2.0 standard deviations). It will take a minimum of eight incrementally includes more quantitative performance meas- observations to estimate the mean delay to within plus or minus 1.5 seconds (a total range or CI of 3.0 seconds) at a ures related to travel time and delay, the necessity to collect and 95 percent confidence level. archive data, develop funding mechanisms, and implement It is rare for an analyst to actually know what the standard policies on data access will become more pressing and agencies deviation will be before conducting the delay or travel-time can be expected to respond positively. measurements. So the usual strategy is to take 10 measure- Planners conducting travel-time-related analysis on in- ments of the delay (or travel time), and then compute the strumented highways should first explore what the TMC has sample standard deviation from those 10 measurements. The to offer in terms of data before instituting a primary data col- confidence interval for the mean is then computed, and if lection effort. If a planning agency anticipates regular need the computed confidence interval is satisfactory (e.g., less than for this type of data, it would be cost-effective to work with the desired precision for the mean), no more measurements the TMC to develop general policies and protocols for are required. If the computed confidence interval is unsatis- obtaining TMC data. Once the planner has established a factory (e.g., too large), additional measurements of delay (or data collection plan, the following steps will provide useful travel time) are made. Equation 3.1 is used to compute the guidance in collecting travel-time data on roadways covered total number of measurements required. The required num- by the TMC. ber of additional measurements is the difference between the total computed per Equation 3.1 and the number of measure- Step 1. Identify TMC(s) ments already completed (in this example, 10 measurements and Traffic Manager(s) already would have been completed). The first step is to identify the relevant TMC and agency operator collecting data for the desired geographic area and 3.4 Collecting Data from TMCs facility types. Some major urban areas have more than one Collecting data from TMC requires some special consider- TMC. If you are unsure where to begin, the state depart- ations. Most TMCs were created to monitor existing traffic ment of transportation is a good default starting point. conditions for the purpose of relaying information to the State-operated TMC may focus exclusively on freeways,
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31 Desired Precision (CI/S) Desired Confidence Minimum Observations 0.5 99% 130 0.5 95% 83 0.5 90% 64 1.0 99% 36 1.0 95% 23 1.0 90% 18 1.5 99% 18 1.5 95% 12 1.5 90% 9 2.0 99% 12 2.0 95% 8 2.0 90% 6 CI = desired confidence interval in seconds; and S = standard deviation in seconds. Exhibit 3.5. Minimum observations to obtain desired confidence interval. while locally operated TMCs often focus exclusively on city data. Access to archived data will allow the planner to collect and county streets. Once all relevant TMCs have been iden- needed data in one pass. Access only to real-time data will tified, contact the traffic manager for each, who is usually require an extended collection effort, the length of which is located in the Operations Department. determined by your sampling plan. If access is granted, pro- ceed to Step 4. If traffic managers are unable or unwilling to allow data access, ask if they share their data with Value Added Step 2. Communicate Data Collection Resellers (VAR) and, if so, whom. You may be able to obtain Needs data from VARs for a fee. If not, the planner will probably Determining the suitable data for your planning applica- need to institute a primary data collection effort. tion will require direct contact with the TMC Traffic Manager or operations staff. Calculating performance measures will Step 4. Acquire Data require real-time traffic surveillance data for the roadway seg- ments and time periods that are the focus of your analysis. The analyst can be quickly buried under the enormous This data must be archived (i.e., one or more days of data amounts of detailed data available from TMCs, and should stored in readily retrievable format) to be useful. Ideally, the therefore establish in advance what locations, what times of TMC will collect and archive speed and traffic flow data. If so, day, what days of the week, and which weeks the data will be proceed to the next step regarding data access policies. If not, collected. The analyst should consult with the TMC staff ask whether other agencies or private companies collect speed regarding the reliability of the traffic detectors and whether and traffic flow data for the study area roadway segments. If certain locations tend to be more reliable than others. There not, the planner will probably need to institute a primary data are some readily available algorithms and techniques that collection effort to generate measures of reliability. can be used to manage these large datasets; the analyst should not sample the real-time data. If the traffic manager or VAR is able and willing to allow Step 3. Ascertain Data Access Policies access to the database or make a copy of unprocessed data, you Determine whether TMC policy allows access to real-time will need to ascertain what computer software is required for and/or archived data for downloading, and whether the copying and/or reading the data. For answers to this question, agency will provide a copy of the unprocessed data for the the analyst may need to speak with IT personnel at the TMC. roadway segments and times you specify. Asking for verbatim Depending on the TMC and the data requirements of the copies of unprocessed data bypasses most institutional prob- analyst, data acquisition may be feasible over the Internet or lems for agencies lacking policies and protocols for sharing require the installation of specialized equipment at the