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The transit skimming procedure creates skim tables for five time periods, three access/egress combinations, and six line-haul mode combinations.

The five time periods are: (i) early AM, before 6 am; (ii) AM peak period, 6 am to 10 am; (iii) midday, 10 am to 3 pm; (iv) PM peak period, 3 pm to 7 pm; and (v) evening, after 7 pm.

The three access/egress combinations are: (i) walk/transit/walk; (ii) drive/transit/walk; and (iii) walk/transit/drive.

The six line-haul mode combinations are: (i) long-haul premium or commuter rail; (ii) medium-haul premium or heavy rail (BART) (iii) medium-haul basic or express bus; (iv) short-haul premium or light rail or ferry; (v) short-haul basic or local bus; and (vi) a generic transit path used to calculate accessibility. Some notes about the line-haul mode combinations:

• Light rail and ferry are grouped together. This was done to reduce the number of skims that need to be created. Because light rail and ferry do not compete with each other, travelers in corridors with light rail are presented with the light rail choice and travelers in corridors with ferry are presented with the ferry choice.

• What is the "generic transit path"? The generic skims are used by Accessibility.job to create skims/accessibility.csv which is used by the auto ownership model. They do not necessarily represent the best transit impedance for each OD; they represent paths found in which all line-haul modes are treated equally i.e. having equal mode-specific perceived time factors. In comparison, the non-generic line-haul mode skims have mode-specific perceived time factors that are set to be lower for the given line-haul mode than other line-haul modes. The example below, which is an excerpt of TransitSkims.job, compares mode-specific perceived time factors for the generic “all transit treated equally” mode to “local bus” mode.

• Generic transit path example:

; all transit treated equally
; mode-specific perceived time factors
; support loc bus, exp bus, ferry, lt rail, hvy rail, com rail
token_modefac = 'modefac = 9*2.0, 70*1.0, 20*1.0, 10*1.0, 10*1.0, 10*1.0, 10*1.0'

The mode-specific perceived time factors are the same (1.0) for all mode all modes, except for support -- access/egress/transfer -- which has a factor of 2.0.

• Local bus example:

; local bus or short-haul basic
; mode-specific perceived time factors
; support loc bus, exp bus, ferry, lt rail, hvy rail, com rail
token_modefac = 'modefac = 9*2.0, 70*1.0, 20*1.5, 10*1.5, 10*1.5, 10*1.5, 10*1.5' 

The mode-specific perceived time factor for local bus is 1.0, and they are higher (1.5) for other modes.

Note that the skims themselves represent actual time and perceived time is only used for path building.

The procedure outputs a 16-table skim for each time-of-day, access/egress, and line-haul mode combination, containing the following:

Name	Definition	Notes
ivt	Transit in-vehicle time, time (minutes x 100)
iwait	Initial wait time, time (minutes x 100)
xwait	Transfer wait time, time (minutes x 100)
wacc	Walk access time (mode 1), time (minutes x 100)	Walk from centroid to auxiliary rail node/bus stop
waux	Auxiliary walk time (modes 3, 4, and 5), time (minutes x 100)	Transfer between walk auxiliary node and station platform node
wegr	Walk egress time (mode 6), time (minutes x 100)	Walk from auxiliary rail node/bus stop to centroid
dtime	Drive access and/or egress time (modes 2 or 7), time (minutes x 100)
ddist	Drive access and/or egress distance (modes 2 or 7), distance (miles x 100)
fare	Fare, cents (2000$)
boards	Boardings, number
ivtLOC	Local bus in-vehicle time (modes 10 through 79), time (minutes x 100)
ivtLRF	Light rail and/or ferry in-vehicle time (modes 100 through 119), time (minutes x 100)
ivtEXP	Express bus in-vehicle time (modes 80 through 99), time (minutes x 100)
ivtHVY	Heavy rail in-vehicle time (modes 120 through 129), time (minutes x 100)
ivtCOM	Commuter rail in-vehicle time (modes 130 through 139), time (minutes x 100)
ivtFerry	Ferry in-vehicle time (modes 100 through 109), time (minutes x 100)

A complete list of transit mode definitions can be found in TransitModes.

More information about walk access time, walk egress time and auxiliary walk time can be found in TransitNetworkCoding.