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	5,113,323	2,245,780	101.56	41,487	20,583
DAL	63.37	7,520,917	4,603,509	44.81	62,671	43,279
FLL	54.74	3,190,320	2,061,675	43.42	24,904	17,364
STL	43.81	4,840,496	3,365,799	29.88	38,851	29,912
CMH	41.04	1,474,288	1,045,275	29.97	12,198	9,385
SJC	38.54	3,354,556	2,421,374	30.98	29,579	22,583
BNA	35.47	4,343,550	3,206,233	25.15	35,266	28,178
SMF	34.47	3,265,804	2,428,597	25.71	27,848	22,152
AUS	33.67	2,844,141	2,127,806	27.35	23,209	18,225
BUR	33.15	1,972,878	1,481,716	26.19	18,560	14,708
BOS	32.56	1,444,082	1,089,340	24.94	11,548	9,243
LGA	32.38	1,375,358	1,038,935	21.41	10,871	8,954
PDX	30.51	1,754,529	1,344,352	30.39	14,552	11,160
DEN	29.49	9,238,404	7,134,232	20.76	68,481	56,708
MCO	26.72	5,556,692	4,385,050	17.37	42,100	35,869
RDU	26.23	1,268,934	1,005,288	17.51	10,529	8,960
SAN	25.61	4,625,895	3,682,630	17.72	37,940	32,228
OAK	25.47	4,627,213	3,687,828	18.68	39,981	33,687
TPA	23.73	3,548,689	2,868,148	16.70	28,437	24,368
MSY	21.37	2,411,943	1,987,324	15.41	20,268	17,562
MCI	17.42	3,023,687	2,575,160	15.03	25,543	22,206
LAX	16.55	4,950,684	4,247,772	10.52	42,045	38,043
PHX	15.69	7,702,684	6,658,313	9.08	61,848	56,698
BWI	15.05	8,804,760	7,653,099	7.66	70,619	65,597
ONT	14.89	1,383,982	1,204,610	8.67	11,864	10,917
SFO	14.76	1,842,243	1,605,261	10.20	16,256	14,752
SAT	13.23	1,978,043	1,746,963	6.40	16,595	15,597
PHL	11.30	1,117,982	1,004,491	-0.07	8,526	8,532
MKE	11.10	1,514,116	1,362,819	0.55	12,194	12,127
HOU	10.35	6,306,075	5,714,564	0.22	51,900	51,784
SLC	8.38	1,285,523	1,186,180	6.37	10,961	10,305
MDW	6.01	10,084,762	9,512,933	3.19	80,264	77,783
SEA	5.45	1,560,121	1,479,434	2.17	12,384	12,121
LAS	3.41	9,003,001	8,706,133	-0.92	72,975	73,655
SNA	-1.72	1,847,277	1,879,587	-2.96	15,874	16,358
ABQ	-14.35	1,342,401	1,567,220	-16.81	11,538	13,870

Data is for departing flights only. All data obtained from T-100 Domestic segments

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