TST calculation
The best way to calculate a TST is to analyse the history of transportation arrival time (real data) and not doing assumptions. It is highly recommended to start as soon as possible to log your transportation arrival times even before you plan to implement VRO to have enough data when the day will come.
TST can change from time to time:
- Seasons have sometimes a big impact on TST. Toyota in USA for example, increases by 24 hours its TST during winter to protect its plants against delays due to snowstorms. This increase is done in one time but it is planned ahead to reduce its impact on suppliers schedule.
- road traffic during holidays may be a reason to temporary increase TST;
- weekends may have the same effects;
- Specific hours during the day in large cities either.
If your area or if a transportation route cross an area impacted by snow you need to understand seasons impact. In this case you need at least one year of data collection to have a good idea about maximum delays and occurrences. Weather forecast reliability is also important. If you can modify your TST from week to week based on good forecasts you can avoid a high TST for a complete season.
- TST cannot be generally higher than the time between 2 deliveries. If a truck has a break down, the worse case is that you have to wait until the next truck arrives. This rule may not be true when the frequency is high. For example, if you have deliveries every hours, a road accident can impact multiple deliveries;
- TST may be eliminated.
Transportation Safety Time can be splitted in:
- Delivery Safety Time (DST);
- Safety Transit Time (STT) .
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