Guide to Calculating Mobility Management Benefits Victoria Transport Policy Institute
Care is needed when calculating the cumulative impacts of multiple strategies. Some factors overlap. For example, commute trip reduction programs often include guaranteed ride home services and parking cash out. It would be wrong to add these strategies together. When evaluating the impacts of factors that overlap, use professional judgment to determine how much of each to apply.
Total impacts are multiplicative not additive, because each additional factor applies to a smaller base. For example, if one strategy reduces trips by 20%, and a second strategy reduces trips an additional 15%, their combined effect is calculated 80% x 85% = 68%, a 32-point reduction, rather than adding 20% + 15% = 35%. This occurs because the 15% reduction applies to a base that is already reduced 20%. If a third strategy reduces demand by another 10%, the total reduction provided by the three factors together is 38.8% (calculated as (100% - [80% x 85% x 90%]) = (100% - 61.2%) = 38.8%), not 45% (20% + 15% + 10%).
However, some strategies have synergistic effects (total impacts are greater than the sum of their individual impacts). For example, by itself, providing rideshare matching may reduce trips to a particular location by just 5%, and by itself a parking fee may reduce trips by just 10%, but together they may reduce demand by 20% because they provide complementary incentives to change.