Why Your Estimated Travel Time Was Consistently Too Short
A consistently, not just occasionally, underestimated travel time usually points to a systematic gap — using an assumed average speed that's higher than what's actually achievable on the specific route (due to speed limits, terrain, or typical traffic), or not building in any buffer at all for stops and minor delays that occur on virtually every real trip.
A pattern of consistently arriving later than estimated (rather than an occasional one-off delay) points to a specific, fixable gap in how the estimate itself is being calculated.
An unrealistic assumed average speed
Using a speed assumption based on posted speed limits alone, without accounting for realistic average speed given typical traffic, terrain, or road type on the specific route, consistently overstates achievable speed — checking actual historical travel times for a familiar route and comparing them against the assumed speed used in the calculation reveals whether this is the source of a consistent underestimate.
No buffer for stops and minor delays
As covered in the optimistic-estimate guide, a basic calculation with zero buffer for any stop or delay will consistently underestimate real trips, since virtually every real trip includes at least some of these factors — building in a standard buffer as a default practice, rather than treating each trip's delays as a one-off surprise, corrects this consistent pattern.
Recalibrating based on actual experience
For a frequently traveled route, comparing several actual trip durations against the calculated estimate reveals a personal, route-specific correction factor — using this real, observed pattern going forward gives a more consistently accurate estimate than continuing to rely on the same generic assumption that's repeatedly proven too optimistic.