Southwest has been the dominate carrier at STL since AA wound down the former TWA hub and they continue to grow their departures at the airport pretty much each year. In 2018, they served 46 domestic destinations with regular service as well as a handful of international routes.

What does that growth look like? When looking at number of departures, Southwest flew a total of 29,912 departures in 2014 serving 3,365,799 passengers and in 2018 those numbers had grown to 38,851 departures and 4,840,496 passengers. Growth (in passengers) can come from adding new routes, adding more frequency in existing routes, or putting larger planes on a route. Since Southwest only uses two 737 configurations, the 700 (143 passengers) or 800 (175 passengers) and their MAX variants (although those are currently not is service), they have some hard limits on up sizing the airplanes on a route, so most of the growth has come from adding new routes and adding frequency, making departures an easy way to look at growth.

Since 2014, Southwest added 12 new routes mostly to West Coast and smaller Midwest cities (CHS, BDL, SJC, SMF, OAK, PDX, PIT, AUS, ICT, LIT, DSM, CLE) while only dropping one route (Grand Rapids). They did start and subsequent stop a route to John Wayne airport in Southern California, too. Other than Portland, the routes all were new destinations for STL or replacing existing service cuts (in the case of PIT and CLE).

On 15 existing routes Southwest added significant frequency, while only lowering frequency on two routes (other than GRR). Both of those routes were to Midway-Chicago and Dallas-Love Field. Because of all the new routes and increased frequencies, it make sense that Southwest could reduce frequencies to those two hubs that serve a lot of connecting traffic. The ability to fly direct should lower the need for STL passenger to connect through either Dallas or Chicago. So the reduction in flights to these hubs may actually be a good sign that STL can serve it's passengers better without needing to connect as much. Overall, the route increases were not concentrated in any geographic areas. There was frequency growth to destinations in on both East and West Coasts, the Midwest, the South and a couple of spots in between. The below charts show the changes in departures for all routes at the end of 2018

Increased: Routes with at least 100 more flights in 2018, No Change: Within +/- 100 flight from 2014-2018. Decreased: Routes with at least 100 less flights in 2018

Left: Individual routes 5year departure changes at STL. Right: Indivdual ROutes stacked to show total departures at STL

In total, the growth in new routes and frequencies has expanded passenger's options with only limited pull back from a few hubs. Unless you are frequent Grand Rapids visitor, the change seems positive all around. It will be interesting to see if the growth at STL will continue at the same rate in 2019 for Southwest with the 737-MAX grounding putting constraints on their expansion.

While STL is a growing market for Southwest, how has it compared to the growth in the rest of Southwest's network? We compared the 5 year growth in passengers and departures for all of Southwest's destinations. When focusing on airports where Southwest flew at least 1 million domestic travelers on departing flights 2014, STL 5 yr passenger growth was one of the highest at Southwest. There were 36 airports that Southwest had at least 1 million departing customers in 2014 and all but two of those saw growth between 2014 and 2018. St. Louis' 43.8% increase in total passengers during those five years ranked 4th overall behind only Atlanta, Dallas, and Fort Lauderdale.

Below are the numbers for the 36 airport that were looked at and if you want to dig into the individual route numbers you can use the Airline Data Browser tool (although it has a limited selection of airports to choose from at this time).

Airport	% Passenger Growth	2018 Passengers	2014 Passengers	% Departure Growth	2018 Departures	2014 Departures
ATL	127.69	5113323	2245780	101.56	41487	20583
DAL	63.37	7520917	4603509	44.81	62671	43279
FLL	54.74	3190320	2061675	43.42	24904	17364
STL	43.81	4840496	3365799	29.88	38851	29912
CMH	41.04	1474288	1045275	29.97	12198	9385
SJC	38.54	3354556	2421374	30.98	29579	22583
BNA	35.47	4343550	3206233	25.15	35266	28178
SMF	34.47	3265804	2428597	25.71	27848	22152
AUS	33.67	2844141	2127806	27.35	23209	18225
BUR	33.15	1972878	1481716	26.19	18560	14708
BOS	32.56	1444082	1089340	24.94	11548	9243
LGA	32.38	1375358	1038935	21.41	10871	8954
PDX	30.51	1754529	1344352	30.39	14552	11160
DEN	29.49	9238404	7134232	20.76	68481	56708
MCO	26.72	5556692	4385050	17.37	42100	35869
RDU	26.23	1268934	1005288	17.51	10529	8960
SAN	25.61	4625895	3682630	17.72	37940	32228
OAK	25.47	4627213	3687828	18.68	39981	33687
TPA	23.73	3548689	2868148	16.70	28437	24368
MSY	21.37	2411943	1987324	15.41	20268	17562
MCI	17.42	3023687	2575160	15.03	25543	22206
LAX	16.55	4950684	4247772	10.52	42045	38043
PHX	15.69	7702684	6658313	9.08	61848	56698
BWI	15.05	8804760	7653099	7.66	70619	65597
ONT	14.89	1383982	1204610	8.67	11864	10917
SFO	14.76	1842243	1605261	10.20	16256	14752
SAT	13.23	1978043	1746963	6.40	16595	15597
PHL	11.30	1117982	1004491	-0.07	8526	8532
MKE	11.10	1514116	1362819	0.55	12194	12127
HOU	10.35	6306075	5714564	0.22	51900	51784
SLC	8.38	1285523	1186180	6.37	10961	10305
MDW	6.01	10084762	9512933	3.19	80264	77783
SEA	5.45	1560121	1479434	2.17	12384	12121
LAS	3.41	9003001	8706133	-0.92	72975	73655
SNA	-1.72	1847277	1879587	-2.96	15874	16358
ABQ	-14.35	1342401	1567220	-16.81	11538	13870

Data is for deprting flights olny. All data obtained from T-100 Domestic segments

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