Evaluation of Travel Time Methods to Support Mobility Performance Monitoring:
World Trade Bridge (Page 4 of 5)
Data Quality Steps
At the end of each day of data collection, the supervisor would collect the PDAs and download the data into the field laptop computer where it was stored on the hard drive. The data would be examined for any anomalies and transferred across the Internet to a secondary location for backup purposes. The IB-1 and IB-2 data would be merged together and license plates from the two locations would be "matched" using a spreadsheet developed in Microsoft Excel. As it is easy to mistake certain characters, particularly letters that looked like numbers, the license plate data was pre-processed. All 'I's were replaced with '1's; all 'O's, 'D's, and 'Q's were replaced with '0's; all 'S's were replaced with '5's; and all 'Z's were replaced with '2's. In addition, the data collectors were instructed to always use '1's for 'I's and '0's for 'O's (i.e., to use the digit, rather than the letter).
Occasionally, collectors would be unsure about a license plate and would append "QQQ" to their entry. This would typically occur when several trucks passed the collector in rapid succession or if one truck blocked the license plate of another and he or she could only manage a quick glimpse. This would allow the supervisor to search the downloaded data for a potential match by using the travel times of other trucks that were recorded in the same general time frame. During this process, the supervisor could also identify the few records in which the data collector forgot to press "ENTER" after recording a license plate before recording the next one. These ten-character entries could be split into two and the time for the first interpolated from the adjacent entries if they were less than a minute or so apart.
Data post-processing also included a step to identify any anomalies in the data, including outliers. Outliers, records that indicated travel times significantly greater than typical for that time period, were most often caused by recording the license plate of a vehicle only some of the time as it made repeated trips across the border during a single day. This is because the matching algorithm uses the most recent time at the #1 position when matching to a record from a #2 location. For example, if the vehicle was recorded as it headed from Mexico to the U.S. early in the morning, later returned to Mexico, was missed as it re-entered the U.S. later in the day, and then recorded on its subsequent return to Mexico, the #1 time from its first trip would be matched with it #1 time from the first trip (for a valid travel time) an also matched to the #2 time from its second trip (an invalid travel time). This invalid travel time would be easily identified by manual inspection of the data, aided by highlighting those travel times about a certain, variable threshold.
Freight Delay Analysis
The measure for the freight transportation system at international roadway border crossings is travel delay per truck trip through the first inspection point in the import country. Delay is measured relative to the travel time at low volume conditions, which will allow the processing time of the inspection to be accommodated outside of the measure. Estimating the average delay per truck for each hour where congestion is present, and then applying the average hourly truck volume produces an estimate of total delay.
The average delay per truck for each hour is the difference between the travel time at low volume conditions and the travel time each hour. Travel time is also affected by the number of open inspection booths and this information was recorded on all days as it changed. To determine the average travel time for each road segment, the matched license plate data in the database is used. The number of matches are noted for statistical analysis and the travel time is noted for each hour. The travel time for each truck was assigned to the hour when they passed through the primary Customs inspection location.
The data are presented in Tables 11 through 15. The columns illustrate the key elements for estimating delay:
- No Delay Travel Time – The time through the system at low volume conditions. For this report, the value used was that of the lowest hourly travel time in that direction for each three-day data collection period.
- Average Number of Open Booths – The average number of primary Customs commercial vehicle inspection booths open and available for processing trucks. This figure is not used to compute delay but is useful to help understand the relationship between booths, traffic volume, and delay.
- Number of Matched Vehicles – The number of vehicle observation used to estimate the travel time for each hour.
- Average Travel Time – The amount of travel time from entry to exit for trucks entering the system each hour (use the time the vehicle passes the advance point as the determinant of the time period label).
- Delay per Trip – The difference between the average travel time and the "no delay" time.
- Average Traffic Volume – The average hourly truck volume for the period being analyzed.
- Total Delay – The product of the hourly truck volume and delay per trip.
Note, as mentioned above, that inbound data for October 30, 2001 are not available due to a problem with the data collection devices that could not be corrected.
| Time Period | (a) "No Delay" Travel Time |
(b) Average No. of Open Booths |
(c) Number of "Matched" Vehicles |
(d) Average Travel Time |
(e) Delay Per Trip (d - a) |
(f) Average Traffic Volume* |
(g) Total Delay (f x e) |
|---|---|---|---|---|---|---|---|
| 8:00–9:00 AM | 12.23 | 6.00 | 23 | 35.67 | 23.44 | 309 | 138.00 |
| 9:00–10:00 AM | 12.23 | 6.00 | 71 | 44.68 | 32.45 | 329 | 426.00 |
| 10:00–11:00 AM | 12.23 | 5.54 | 46 | 32.83 | 20.60 | 288 | 254.84 |
| 11:00–12:00 PM | 12.23 | 6.00 | 46 | 38.35 | 26.12 | 309 | 276.00 |
| 12:00–1:00 PM | 12.23 | 6.00 | 66 | 29.38 | 17.15 | 329 | 396.00 |
| 13:00–2:00 PM | 12.23 | 6.00 | 69 | 14.65 | 2.42 | 391 | 4140 |
| 14:00–3:00 PM | 12.23 | 4.43 | 69 | 19.47 | 7.24 | 309 | 305.67 |
| 15:00–4:00 PM | 12.23 | 5.00 | 70 | 26.38 | 14.15 | 288 | 350.00 |
| 16:00–5:00 PM | 12.23 | 5.94 |
57 | 14.78 | 2.55 | 247 | 338.58 |
| 17:00–6:00 PM | 12.23 | 6.00 | 15 | 12.23 | 0.00 | 309 | 0.00 |
| 18:00–7:00 PM | 12.23 | 5.67 | 23 | 35.67 | 23.44 | 370 | 130.41 |
* Average traffic volume estimated by applying TTI traffic count hourly inbound truck traffic percentages from 8/15/00 and 8/16/00 to U.S. Customs data for 10/31/01
| Time Period | (a) "No Delay" Travel Time |
(b) Average No. of Open Booths |
(c) Number of "Matched" Vehicles |
(d) Average Travel Time |
(e) Delay Per Trip (d - a) |
(f) Average Traffic Volume* |
(g) Total Delay (f x e) |
|---|---|---|---|---|---|---|---|
| 8:00–9:00 AM | 12.23 | 6.00 | 91 | 22.50 | 10.27 | 301 | 546.00 |
| 9:00–10:00 AM | 12.23 | 5.05 | 102 | 20.35 | 8.12 | 321 | 515.10 |
| 10:00–11:00 AM | 12.23 | 5.00 | 90 | 37.68 | 25.45 | 281 | 450.00 |
| 11:00–12:00 PM | 12.23 | 5.00 | 80 | 55.02 | 42.79 | 301 | 400.00 |
| 12:00–1:00 PM | 12.23 | 5.81 | 97 | 58.63 | 46.40 | 321 | 563.57 |
| 13:00–2:00 PM | 12.23 | 5.99 | 79 | 53.32 | 41.09 | 381 | 473.21 |
| 14:00–3:00 PM | 12.23 | 5.99 | 77 | 36.25 | 24.02 | 301 | 461.23 |
| 15:00–4:00 PM | 12.23 | 5.44 | 89 | 29.13 | 16.90 | 281 | 484.16 |
| 16:00–5:00 PM | 12.23 | 5.00 | 78 | 17.73 | 5.50 | 241 | 390.00 |
| 17:00–6:00 PM | 12.23 | 5.00 | 37 | 20.75 | 8.52 | 301 | 185.00 |
| 18:00–7:00 PM | 12.23 | 6.00 | 91 | 22.50 | 10.27 | 361 | 546.00 |
* Average traffic volume estimated by applying TTI traffic count hourly inbound truck traffic percentages from 8/15/00 and 8/16/00 to U.S. Customs data for 11/1/01
| Time Period | (a) "No Delay" Travel Time |
(b) Average No. of Open Booths |
(c) Number of "Matched" Vehicles |
(d) Average Travel Time |
(e) Delay Per Trip (d - a) |
(f) Average Traffic Volume* |
(g) Total Delay (f x e) |
|---|---|---|---|---|---|---|---|
| 8:00–9:00 AM | 2.22 | 2.90 | 16 | 3.07 | 21:16 | 64 | 46.40 |
| 9:00–10:00 AM | 2.22 | 3.65 | 30 | 4.97 | 20:16 | 129 | 109.50 |
| 10:00–11:00 AM | 2.22 | 4.00 | 43 | 5.60 | 19:16 | 193 | 172.00 |
| 11:00–12:00 PM | 2.22 | 4.00 | 56 | 7.60 | 18:16 | 236 | 224.00 |
| 12:00–1:00 PM | 2.22 | 3.04 | 39 | 8.35 | 17:16 | 236 | 118.56 |
| 13:00–2:00 PM | 2.22 | 3.00 | 46 | 17.37 | 16:16 | 301 | 138.00 |
| 14:00–3:00 PM | 2.22 | 3.33 | 41 | 25.63 | 15:16 | 343 | 136.53 |
| 15:00–4:00 PM | 2.22 | 5.91 | 102 | 33.03 | 14:16 | 386 | 602.82 |
| 16:00–5:00 PM | 2.22 | 7.00 | 89 | 20.48 | 13:16 | 386 | 623.00 |
| 17:00–6:00 PM | 2.22 | 7.00 | 50 | 19.45 | 12:16 | 408 | 350.00 |
| 18:00–7:00 PM | 2.22 | 2.90 | 16 | 3.07 | 11:16 | 472 | 46.40 |
*Average traffic volume estimated by applying TTI traffic count hourly outbound truck traffic percentages from 8/15/00 and 8/16/00 to City of Laredo data for 10/30/01
| Time Period | (a) "No Delay" Travel Time |
(b) Average No. of Open Booths |
(c) Number of "Matched" Vehicles |
(d) Average Travel Time |
(e) Delay Per Trip (d - a) |
(f) Average Traffic Volume* |
(g) Total Delay (f x e) |
|---|---|---|---|---|---|---|---|
| 8:00–9:00 AM | 2.22 | 3.58 | 37 | 2.22 | 0 | 66 | 0.00 |
| 9:00–10:00 AM | 2.22 | 3.58 | 50 | 5.83 | 3.61 | 133 | 480.13 |
| 10:00–11:00 AM | 2.22 | 4.00 | 79 | 17.10 | 14.88 | 199 | 2961.12 |
| 11:00–12:00 PM | 2.22 | 4.00 | 60 | 29.42 | 27.2 | 244 | 6636.80 |
| 12:00–1:00 PM | 2.22 | 4.00 | 34 | 39.43 | 37.21 | 244 | 9079.24 |
| 13:00–2:00 PM | 2.22 | 3.73 | 62 | 45.33 | 43.11 | 310 | 13364.10 |
| 14:00–3:00 PM | 2.22 | 4.17 | 77 | 61.00 | 58.78 | 354 | 20808.10 |
| 15:00–4:00 PM | 2.22 | 7.28 | 106 | 44.42 | 42.2 | 399 | 16837.80 |
| 16:00–5:00 PM | 2.22 | 8.98 | 85 | 10.87 | 8.65 | 399 | 3451.35 |
| 17:00–6:00 PM | 2.22 | 8.00 | 30 | 13.88 | 11.66 | 421 | 4908.86 |
| 18:00–7:00 PM | 2.22 | 3.58 | 37 | 2.22 | 0 | 487 | 0.00 |
* Average traffic volume estimated by applying TTI traffic count hourly outbound truck traffic percentages from 8/15/00 and 8/16/00 to City of Laredo data for 10/31/01
| Time Period | (a) "No Delay" Travel Time |
(b) Average No. of Open Booths |
(c) Number of "Matched" Vehicles |
(d) Average Travel Time |
(e) Delay Per Trip (d - a) |
(f) Average Traffic Volume* |
(g) Total Delay (f x e) |
|---|---|---|---|---|---|---|---|
| 8:00–9:00 AM | 2.22 | 2.78 | 25 | 2.50 | 0.28 | 65 | 180.70 |
| 9:00–10:00 AM | 2.22 | 2.96 | 39 | 4.30 | 2.08 | 129 | 381.84 |
| 10:00–11:00 AM | 2.22 | 3.00 | 40 | 10.70 | 8.48 | 194 | 582.00 |
| 11:00–12:00 PM | 2.22 | 4.01 | 46 | 20.52 | 18.3 | 237 | 950.37 |
| 12:00–1:00 PM | 2.22 | 4.30 | 62 | 8.72 | 6.5 | 237 | 1019.10 |
| 13:00–2:00 PM | 2.22 | 4.32 | 54 | 4.37 | 2.15 | 302 | 1304.64 |
| 14:00–3:00 PM | 2.22 | 4.26 | 55 | 14.42 | 12.2 | 345 | 1469.70 |
| 15:00–4:00 PM | 2.22 | 6.85 | 78 | 17.85 | 15.63 | 388 | 2657.80 |
| 16:00–5:00 PM | 2.22 | 7.85 | 87 | 5.05 | 2.83 | 388 | 3045.80 |
| 17:00–6:00 PM | 2.22 | 8.00 | 33 | 6.98 | 4.76 | 410 | 3280.00 |
| 18:00–7:00 PM | 2.22 | 2.78 | 25 | 2.50 | 0.28 | 474 | 1317.72 |
* Average traffic volume estimated by applying TTI traffic count hourly outbound
percentages from 8/15/00 and 8/16/00 to City of Laredo data for 11/1/01
As mentioned above, data was collected all trucks and for trucks without cargo because they were specifically treated differently at the Mexican import and export booths. When delay times are compared for unloaded trucks (including bobtails) and loaded trucks average delay time do vary. Table 16 summarizes the weighted average travel times between empty trucks and loaded flights.
| Direction | Loaded or Empty | Weighted Travel Time |
|---|---|---|
| Inbound | Empty Trucks | 21.50 minutes |
| Outbound | Empty Trucks | 2.63 minutes |
| Inbound | Loaded Trucks | 32.39 minutes |
| Outbound | Loaded Trucks | 19.28 minutes |
