Skip to content
Upcoming courses

Required safe egress time

Tools
From Fire Safety Academy Wiki
Contents
  1. 1Definition and terminology
  2. 2Components
  3. 3Calculation
  4. 4Worked example
  5. 5Methods
  6. 6RSET for different locations
  7. 7Comparison with ASET and safety margins
  8. 8Limits and uncertainty
  9. 9See also
  10. 10Notes
  11. 11References

The required safe egress time (RSET) is the time occupants need to leave a room or building in a fire. It is counted from ignition until all of them have left that space or reached a place of safety.[1][2] It includes not only the walking time but also everything that happens before people start to move, such as detection, the alarm and the time occupants take to respond.[2] RSET is one half of the comparison at the centre of performance-based life safety design. The other half is the available safe egress time (ASET), the time until conditions in the space become untenable.[3]

A design is acceptable for life safety when ASET is longer than RSET, usually with a safety margin or safety factor.[2] The RSET is calculated by adding the detection time, the notification time, the pre-movement time and the movement time.[1] The movement time in turn depends on the walking speed of the occupants and on the flow capacity of doors and stairs.[4] Because human behaviour before movement is hard to predict, and simple models assume ideal movement, a calculated RSET is best seen as a minimum time rather than a prediction of the real evacuation.[5]

Definition and terminology

In New Zealand, C/VM2 (second edition, 2025) defines the RSET as the calculated time between ignition of the design fire and the moment when all occupants of a specified room or location have left it.[1] The commentary to the first edition adds that RSET is generally the time for all occupants to reach a place of safety. It can also be defined for one space, such as the room of fire origin.[6] The Introduction to Fire Safety Engineering describes RSET as the time needed to leave the room or building, and stresses that its calculation rests on assumptions about how people will react.[2]

The International Fire Engineering Guidelines (2005) use the expansion "Required Safe Evacuation Time" for the same abbreviation.[7] They define it as the time from fire initiation needed for occupants to reach a place of safety, which is then compared with ASET.[7][8] The guidelines note that the times may vary with the location of the occupants relative to the fire.[7]

Components

Most sources split the RSET into four or five consecutive periods. C/VM2 uses a detection time, a notification time, a pre-travel activity time and a travel or flow time.[1] The Australian data sheets for the Fire Safety Verification Method use the same four parts.[9] Nelson and Mowrer split the time before movement into two parts: until occupants decide to act, and from that decision until evacuation starts.[3] The International Fire Engineering Guidelines speak of a cue period, a response period, a delay period and a movement period.[10]

Timeline from ignition: detection, notification, pre-movement and movement make up RSET; a longer ASET bar ends at untenable conditions.
The components of the required safe egress time and its comparison with the available safe egress time. Based on Ministry of Business, Innovation and Employment (2025b), p. 26, and Van Rossum (2027), §7.1.

The detection time runs from ignition until an automatic system or a person detects the fire. C/VM2 requires it to be found from deterministic modelling of the detectors or sprinklers, using fixed detector properties regardless of the products to be installed.[1][11] Methods to calculate it are described under detector activation time. The notification time covers the delay between detection and the alarm reaching the occupants. C/VM2 takes it as 30 s for standard evacuation strategies, and longer where the alarm is first investigated by staff.[12]

The pre-movement time follows: the time between the alarm or a cue and the moment occupants begin to move.[13] In residential drills studied by Proulx, three-quarters of the evacuation time was spent before moving.[14] The last component is the movement time. Proulx distinguishes the time needed to go from a starting point to a destination such as a place of safety from the flow time. The flow time is the time a crowd needs to pass one element of the route, such as a door or stair.[15] The flow time follows from the number of people and the exit capacity of the usable width.[15]

Calculation

C/VM2 (second edition, 2025) calls its standard method the simple hydraulic model for RSET.[1] The time before movement is the sum of the detection, notification and pre-travel times. The movement time is either the travel time to the exit or the flow time through the exit, whichever is greater.[1] RSET=td+tn+tpre+max⁡(ttrav,tflow)

Symbol Description Unit
RSET required safe egress time s
td detection time, from ignition until detection s
tn notification time, from detection until the occupants are alerted s
tpre pre-travel activity time, from notification until evacuation begins s
ttrav travel time to the exit s
tflow time spent in queues, controlled by the flow capacity of the exit s

The relation applies to the occupants of one room or location. For the room of fire origin, C/VM2 assumes that the occupants are spread evenly, so that the larger of the travel and flow times governs.[1] For routes through several spaces, the analysis must cover the whole route, so that a restriction closer to the final exit is not missed.[16] The travel speed falls with crowd density and is capped at 1.2 m/s on level routes.[17] The flow through a door is calculated from its effective width and a density of 1.9 persons/m², and is limited to 50 people per minute for each self-closing door leaf.[16][18]

Line chart of horizontal travel speed against crowd density: flat at 1.2 m/s up to 0.54 persons per m², then falling linearly to zero at about 3.8.
Design travel speed on level routes in C/VM2 (second edition, 2025) as a function of crowd density. Based on Ministry of Business, Innovation and Employment (2025b), pp. 32–33.

Like other hydraulic methods, this calculation assumes that all occupants start at the same moment and move without interruptions, and that most are free of disabilities that would slow the group.[3] The Introduction to Fire Safety Engineering adds that a hand calculation requires a homogeneous group of people and a simple geometry.[19] Its eight-step method determines the people per space, the flow capacity of each exit, the distribution of people over the exits and the governing exit. It then adds the walking time of the first person, the flow time of the governing exit and the walking time of the last person to safety.[19]

Worked example

The commentary to the first edition of C/VM2 calculates the RSET for a ballroom of 50 m by 20 m holding 1,000 people.[20] In the sprinklered building, the design fire grows as a fast t-squared fire and is controlled by the sprinklers at a peak of 2,700 kW.[21] Fire modelling shows that the smoke detectors respond after 66 s. The notification time is 30 s. Because the occupants are awake, unfamiliar with the building and alerted by a voice alarm, the pre-travel activity time is 30 s.[22]

The travel time is checked first. At a density of 1 person/m², the travel speed is 1.03 m/s, and the longest route, taken as length plus width of the room, is 70 m. The travel time is therefore 68 s.[23]

For the flow, each of four exit routes takes a quarter of the occupants, or 250 people.[23] A door leaf of 0.95 m has an effective width of 0.65 m. The flow equation gives about 51 people per minute, which is capped at 50, so a pair of doors passes 100 people per minute.[24] The flow time of 150 s is longer than the travel time and governs: RSET=66+30+30+max⁡(68, 150)=276 s For the people who leave through the foyer, the commentary adds another 150 s of flow time, giving 426 s.[25] The modelled tenability limit for carbon monoxide was not reached within the 1,200 s simulated, so ASET exceeds RSET by more than 924 s in the ballroom.[25] The commentary notes that, strictly, the dose must be calculated at the locations of the moving occupants, which may mean using data from different rooms for different periods.[25]

Stacked bars: ballroom 66 + 30 + 30 + 150 = 276 s; via foyer 426 s; a vertical line marks ASET beyond 1,200 s.
RSET in the commentary example: detection, notification, pre-travel and flow time for the ballroom and for the route through the foyer, compared with the ASET. Based on Ministry of Business, Innovation and Employment (2013), pp. 203–206.

The Introduction to Fire Safety Engineering gives a simpler example of the movement part alone. Three hundred people are divided equally over four stairs, and the two narrowest stairs of 1.6 m govern. Each passes 1.2 persons per second, so 75 people need about 63 s.[19] Adding 5 s for the first person to reach the stair and 10 s for the last person to reach safety gives a movement time of 78 s.[19]

Methods

Escape safety can be assessed with methods of increasing complexity. The Introduction to Fire Safety Engineering lists the rules of the Dutch building regulations, a capacity model, hand calculations and evacuation simulations with programs such as Simulex, buildingEXODUS and Pathfinder.[19] The more complex the building, the more detailed the model needs to be, and a simple method can check a complex one.[19]

Evacuation modelling with computer software rebuilds the geometry in three dimensions and fills it with people, often with several occupant profiles.[26] Its output includes the total evacuation time, queues, densities and bottlenecks, which can be used directly in an ASET/RSET comparison.[26] Its main limitation is human behaviour. However convincing the output looks, it reflects the choices made by the modeller, such as which exit people use.[27] The International Fire Engineering Guidelines likewise warn that hydraulic and network models may give shorter times than real evacuations, because they leave out occupant behaviour.[28][29]

Nelson and Mowrer call the result of a hydraulic calculation the modelled evacuation time. The actual time will generally be longer.[30] They express the difference as an apparent evacuation efficiency. It covers delays caused by wardens, stops and restarts where flows merge, individual delays and unbalanced use of exits.[30] Proulx therefore stresses that such calculations give only minimum evacuation times. Some consultants have wrongly presented them as maximum times.[31][5]

RSET for different locations

An RSET is not a single number for a whole building. The International Fire Engineering Guidelines recommend carrying out the analysis for each enclosure, such as a room or a mall, or for groups of enclosures such as a floor or the whole building.[32] The analysis is repeated for each design occupant group defined in the fire engineering brief.[33] Occupants far from the fire usually receive fewer cues and therefore get longer pre-movement times than those in the room of fire origin.[34]

In New Zealand, the challenging fire scenario of C/VM2 (second edition, 2025) places design fires in various locations and requires ASET to exceed RSET for each of them.[35] For some smaller rooms, tenability need not be shown for the occupants of the room of fire origin. This applies, for example, to rooms under 500 m² with fewer than 150 occupants and more than one direction of travel.[35] It must still be shown that a fire in such a room does not threaten the occupants in the rest of the building.[35] Tenability is then assessed in the escape routes over the time occupants need to escape, and at the locations of the occupants throughout the evacuation.[36]

Evacuation strategies also change the RSET from place to place. Where a building uses a phased evacuation, the alarm may sound later in some areas. The Australian data sheets require such delays to be included in the notification time for those areas.[37] In the commentary example above, the occupants who leave the ballroom through the foyer have a longer RSET than those who leave directly outside, because they pass two sets of doors.[24][25]

Comparison with ASET and safety margins

Because many inputs for both ASET and RSET are uncertain, a safety margin is important.[2] The Introduction to Fire Safety Engineering gives two common forms: ASET at least equal to a safety factor times RSET, or ASET minus RSET at least equal to a margin.[2] The more reliable the inputs, the smaller the margin needed, and a sensitivity analysis can show whether the margin is sufficient.[2]

In the Netherlands, the standards for large fire compartments, NEN 6060 and NEN 6079, include an ASET/RSET comparison with safety factors.[2] For new buildings assessed with the rule-of-thumb method and conservative inputs, the factor is 1.5 when RSET is up to 3 minutes and 2.0 when it is longer.[2] With nominal inputs, the factors are 2.0 and 2.5. Existing buildings have lower factors, between 1.0 and 1.5.[2] Proulx suggests that, in a conservative approach, the time available should be at least twice the time required.[31]

Grouped bars of required ASET/RSET factors for new and existing buildings, conservative and nominal inputs, RSET up to and over 3 minutes; values 1.0 to 2.5.
Safety factors on RSET in NEN 6060 and NEN 6079 for the rule-of-thumb method, as reproduced in the Introduction to Fire Safety Engineering. Based on Van Rossum (2027), §7.1.

Other frameworks compare designs rather than set a fixed factor. In Australia, the Fire Safety Verification Method expects the margin of safety, ASET minus RSET, of a proposed performance solution to be at least that of a reference building.[38] In New Zealand, C/VM2 (second edition, 2025) notes that its design fires leave out the incipient phase of a fire, which provides an inherent safety factor for its pre-travel activity times.[34]

Limits and uncertainty

The largest uncertainty usually lies in the time before movement. Human behaviour is one of the least predictable aspects of fire safety, and the data available have not led to generally valid models for pre-evacuation times.[13] The movement part can also be slower than design values suggest. In the first World Trade Center tower in 2001, the median occupant moved down the stairs at about 0.2 m/s, including all interruptions. This is at the slow end of the published values.[39] The flow from the stairs was about 0.5 people per second per door, roughly half the common rule of thumb.[40]

A single RSET also hides the spread between occupants. The Australian data sheets recommend evaluating several scenarios to reflect the different responses of occupant groups and individuals.[38] The International Fire Engineering Guidelines list ways to shorten RSET. These include additional and less ambiguous cues, trained personnel, better signage and improved design of escape routes.[41]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Ministry of Business, Innovation and Employment (2025b), p. 26.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 Van Rossum (2027), §7.1.
  3. ↑ 3.0 3.1 3.2 Nelson and Mowrer (2002), p. 3-367.
  4. ↑ Van Rossum (2027), §7.4.
  5. ↑ 5.0 5.1 Proulx (2002), p. 3-364.
  6. ↑ Ministry of Business, Innovation and Employment (2013), p. 198.
  7. ↑ 7.0 7.1 7.2 National Research Council of Canada et al. (2005), p. 1.8-1.
  8. ↑ National Research Council of Canada et al. (2005), p. 1.8-3.
  9. ↑ Australian Building Codes Board (2020a), Data Sheet D1, p. 242.
  10. ↑ National Research Council of Canada et al. (2005), p. 1.8-2.
  11. ↑ Ministry of Business, Innovation and Employment (2025b), p. 27.
  12. ↑ Ministry of Business, Innovation and Employment (2025b), p. 29.
  13. ↑ 13.0 13.1 Van Rossum (2027), §7.3.
  14. ↑ Proulx (2002), p. 3-348.
  15. ↑ 15.0 15.1 Proulx (2002), p. 3-347.
  16. ↑ 16.0 16.1 Ministry of Business, Innovation and Employment (2025b), p. 33.
  17. ↑ Ministry of Business, Innovation and Employment (2025b), p. 32.
  18. ↑ Ministry of Business, Innovation and Employment (2025b), p. 34.
  19. ↑ 19.0 19.1 19.2 19.3 19.4 19.5 Van Rossum (2027), §7.5.
  20. ↑ Ministry of Business, Innovation and Employment (2013), p. 190.
  21. ↑ Ministry of Business, Innovation and Employment (2013), p. 202.
  22. ↑ Ministry of Business, Innovation and Employment (2013), p. 203.
  23. ↑ 23.0 23.1 Ministry of Business, Innovation and Employment (2013), p. 204.
  24. ↑ 24.0 24.1 Ministry of Business, Innovation and Employment (2013), p. 205.
  25. ↑ 25.0 25.1 25.2 25.3 Ministry of Business, Innovation and Employment (2013), p. 206.
  26. ↑ 26.0 26.1 Van Rossum (2027), §10.1.
  27. ↑ Van Rossum (2027), §10.2.
  28. ↑ National Research Council of Canada et al. (2005), p. 2.8-3.
  29. ↑ National Research Council of Canada et al. (2005), p. 2.8-4.
  30. ↑ 30.0 30.1 Nelson and Mowrer (2002), p. 3-368.
  31. ↑ 31.0 31.1 Proulx (2002), p. 3-363.
  32. ↑ National Research Council of Canada et al. (2005), p. 1.8-7.
  33. ↑ National Research Council of Canada et al. (2005), p. 1.8-4.
  34. ↑ 34.0 34.1 Ministry of Business, Innovation and Employment (2025b), p. 31.
  35. ↑ 35.0 35.1 35.2 Ministry of Business, Innovation and Employment (2025b), p. 56.
  36. ↑ Ministry of Business, Innovation and Employment (2025b), p. 57.
  37. ↑ Australian Building Codes Board (2020a), Data Sheet D1, p. 243.
  38. ↑ 38.0 38.1 Australian Building Codes Board (2020a), Data Sheet D1, p. 241.
  39. ↑ Averill et al. (2005), p. 129.
  40. ↑ Averill et al. (2005), p. 130.
  41. ↑ National Research Council of Canada et al. (2005), p. 1.8-13.

References

  • Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
  • Averill, J. D., Mileti, D. S., Peacock, R. D., Kuligowski, E. D., Groner, N., Proulx, G., Reneke, P. A., & Nelson, H. E. (2005). Occupant behavior, egress, and emergency communications (NIST NCSTAR 1-7). National Institute of Standards and Technology.
  • Ministry of Business, Innovation and Employment. (2013). Commentary for Building Code clauses C1–C6 and Verification Method C/VM2.
  • Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
  • National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Nelson, H. E., & Mowrer, F. W. (2002). Emergency movement. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-367–3-380). National Fire Protection Association; Society of Fire Protection Engineers.
  • Proulx, G. (2002). Movement of people: The evacuation timing. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-342–3-366). National Fire Protection Association; Society of Fire Protection Engineers.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
Retrieved from "https://fire-safety.academy/w/index.php?title=Required_safe_egress_time&oldid=2632"