# Digital Twin for Supply Chain Coordination in Modular Construction
## 4 Case Study
### Results  
As shown in Figure 3, 6 risk types and a total of 12 risk points were detected from logistics simulation in the digital twin. One important fact here is that it is not possible to show all possible logistics risks because risks vary depending on current traffic conditions which change over time. The risks shown in the results give examples of possible risks posed to the case project at a specific time on a particular day. Such hard to predict risks can occur after planning. These risks should be continuously identified and avoided through real-time logistics simulation. In order to indicate the moving direction of the truck, the starting point was marked as 'S', and the destination point was marked as 'D' in the Figure 3. The red line shows the baseline-route and the blue line shows the alternative route (DT-route) found by the digital twin. The first risk involved truck height restrictions. The module carrying truck was designed to be 13 feet 6 inches in height per Washington state's truck height standards and regulations, but there were cases where it could not pass due to bridge or tunnel height limitations. The second risk involved highway width. Because the modules' widths  
Figure 3: Six risk types identified from the logistics risk assessment. exceed 14 feet and the standard lane width in the US is 12 feet, trucks needed to drive on two lanes at the same time. Although trucks can drive this way on most highways with permission from authorities, several ramps have only one lane, making it impossible for the trucks to enter. The third risk arose from bridge weight limits. A truck loaded with a module reaches a maximum weight of 20 tons and the maximum load for several bridges is 17-19 tons, making it impossible for the trucks to enter. The fourth risk was posed by a steep gradient. A dangerous accident may occur if a 20-ton truck enters a slope of more than 10 degrees and this road had a 19-degree slope, so an alternative road that bypasses the slope was searched for. The fifth risk involved the truck's minimum turning radius and a sharp curve. Since the length of the truck reaches a maximum of 50 feet, it cannot rotate unless the curve's inner radius is longer than 28 feet and the outer radius is longer than 40 feet. The digital twin requested an alternative route in which the truck could rotate. The sixth and last risk came from the highway's speed limit. The routing application calculates ETAs based on average travel speeds along the road from historic data, but modular trucks must often observe a different speed limit from normal vehicles. For example, several highways limit the speed of trucks driving in two lanes to 60 mph when it is otherwise 70 mph. Therefore, the digital twin corrected the ETA's value while the truck was driving this section of the route. Both the baseline-route and DT-route were quantitatively compared with a metric of ETA. In both cases, the ETA calculation was performed within a short time of fewer than 2 s, so no comparison was made for ETA calculation time. Since ETA is affected by traffic, the ETA value was calculated according to the delivery start time and drawn as shown in Figure 4a. Since traffic may vary depending on the day of the week, an ETA graph was drawn based on a seven-day average. For the baseline-route, an average of about 156 miles and an ETA of 2 h and 33 min were estimated. For the DT-route, an average of about 173 miles and an ETA of 4 h and 8 min were estimated. (Figure 4a). Since the DT-route ETA was calculated considering potential logistics risks, it can be assumed that this ETA is close to the actual module delivery time. Thus, approximately 1 h and 36 min of delivery time error can be expected in the baseline-route compared to DT-route. Another important implication in Figure 4a is that the predicted ETA value changes over time. A main reason for ETA changes is that traffic conditions change in real time due to factors such as rush hour, temporary accidents, curfews, and construction zones.