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Estimate the added time by subtracting the normal expected trip time from the current end-to-end time for an open detour. For example, if your usual trip is expected to take 48 minutes and a route planner estimates the detour at 1 hour 13 minutes, the detour adds an estimated 25 minutes. Those figures are illustrative; actual travel time can change with traffic, incidents, and closure updates.
How do you calculate the extra time?
Use this comparison:
Estimated extra time = current estimated time on the open alternate route − expected time for the original trip under comparable conditions.
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Compare the same origin and destination and, as far as possible, the same departure time and traffic conditions. The subtraction is a practical planning method, not a universal formula published by transportation agencies.
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If the highway is fully closed, its current trip time cannot serve as the baseline. Use the expected time for the trip without the closure, or compare the full detour journey with the normal expected time for the same trip. If part of the original route remains open, compare complete end-to-end journey times rather than only the detour segment.
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How to get a useful estimate
- Confirm the closure. Check the responsible state or regional 511 service for the affected road, direction, closure limits, incident details, and any estimated reopening time. Coverage and features vary by region. SoCal 511, for example, covers a five-county Southern California region; state services cover their own networks.
- Set a normal-trip baseline. Find the expected time for the same start and destination at roughly the same departure time. A quiet-hour trip is not a fair baseline for a rush-hour detour.
- Find open alternate routes. Use a route planner’s alternate-route feature, then check the candidate route against current official closure and restriction information. ADOT says AZ511 provides delays, crashes, construction, closures, alternate routes, and route travel times.
- Compare full trips. Subtract the baseline from the current detour estimate. Make sure both numbers cover the complete journey, not one route’s total and another route’s detour segment.
- Compare more than minutes. Check added distance, current incidents or congestion, closure certainty, and whether the road is suitable for your vehicle and trip.
- Recheck before departure. Conditions can change. Bay Area 511 says its changeable-sign driving-time averages use GPS probe-vehicle data and update every two minutes; that interval applies to those sign estimates, not all route services.
How can you compare two possible detours?
Use the same origin, destination, and approximate departure time for each route. Treat this as a checklist, not a formal scoring formula: the cited services do not prescribe a common national weighting system.
| What to compare | What to check |
|---|---|
| Estimated end-to-end time | Use the full current trip estimate, not just the portion that bypasses the closure. |
| Extra distance | Consider whether any time saved is worth a substantially longer route. |
| Closure status | Confirm that the route is open using current information from the responsible transportation agency. |
| Live conditions | Check congestion, incidents, construction, and weather that could affect the alternate. |
| Route suitability | Check official conditions and posted restrictions for your vehicle and trip. |
What if a route planner’s estimate seems too optimistic?
A route estimate depends on the service’s data, coverage, and freshness. Wisconsin 511 warns that its routing may not include all road-construction, traffic-incident, or weather-event closures. Cross-check a candidate route against current closure information, especially if missing a restriction would put you on a closed or unsuitable road.
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“Free-flow time” is not necessarily a live detour estimate. AZ511 defines it as the normal expected travel time without congestion, at posted speed limits. Actual traffic around a closure may be slower.
Federal traveler-information regulations in the 2023 Code of Federal Regulations edition set a designed-accuracy requirement of at least 85 percent (or a maximum error rate of 15 percent) for a real-time information program. The same edition sets a timeliness requirement of 10 minutes or less after completion of the calculation for travel-time information along limited-access roadway segments within metropolitan areas. These are program requirements, not a promise that an individual traveler’s estimate will be within 15 percent or that every route planner updates every 10 minutes.
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- Get more situational awareness with alerts for school zones, speed changes, sharp curves and more
- View food, fuel and rest areas along your active route, and see upcoming cities and milestones
- View Tripadvisor traveler ratings for top-rated restaurants, hotels and attractions to help you make the most of road trips
- Directory of U.S. national parks simplifies navigation to entrances, visitor centers and landmarks within the parks
Should you follow an official detour?
Follow signed and officially designated detours where provided. A consumer route planner may not reflect the full operational plan, road restrictions, or traffic-control arrangements. The Federal Highway Administration’s detour guidance describes coordination among highway and public-safety agencies, as well as the work to determine a route and install, maintain, operate, and remove traffic-control devices.
PennDOT’s Traffic Management Center Operations Manual describes “global detours” for major interstate incidents expected to last more than two to four hours, moving mainline traffic to a route able to handle increased capacity. That is agency operations guidance, not a waiting period or rerouting threshold for individual drivers; the manual says time-estimate tools should weigh heavily in the agency’s decision to implement such a detour.
How can you check safely?
Review the route before setting off. Bay Area 511 advises checking before departure, using voice information, or pulling over before checking a smartphone. Do not interact with a phone while driving.
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