Winterset and Indanola might be considered exurban. There distance from the interstates makes them a little harder to commute from, but there may be affordable houses and some jobs, and there is the attraction of living in a small town. One spouse might commute to Des Moines, or perhaps start to do so after they lose a job in the cities.
It's odd though that development has not gone south along I-35 south of I-80. And the NE corner of Madison County has never been developed much, even though it is quite near to Des Moines. The county line seems to be some sort of barrier. It reminds me of the county line between LA and San Bernadino counties, where in LA is the city of Pomona that has been around a long time, and is densely built, and Chino next door which was dairy farms until quite recently. The streets just stopped at the county line.
But I see a bridge is being built across the Raccoon River that might jump start things.
By railroad, Winterset and Indianola were fairly isolated.
1902 Railroad MapThe railroads west are tied to the Mississippi River crossings (Burlington and Rock Island) and Burlington is south of Madison and Warren. If you look closely, there were (proposed) interurban railroads from Des Moines to both Winterset and Indianola (they are in black only). Had they been built, there might have developed a commuter culture. and perhaps suburbanization to the south of Des Moines.
The Rock Island dips down to the east of Des Moines, so the I-35/I-80 loop around the city replaces the direct route through the city. I-80 west of the city follows the railroad, so there are smaller towns that can provide a bit of infrastructure for residential development (water and sewer plants mostly and schools), since the developer would build the lines and roads.
I-35 was developed in virgin territory, US-65 goes south to Indianola, and it appears that US-65 was just to create a north-south route down the middle of the first tier of states west of the Mississippi. I-35 was intended to connect established cities (Duluth-Twin Cities-Des Moines-Kansas City-Wichita-OKC-DFW-Waco-Austin-San Antonio-Laredo) where there had never been a transportation corridor.
The Des Moines and Raccoon rivers appear to have guided development towards the west. While the Capitol is to the east of the river, the downtown area is to the west, and there is enough flooding to push residential areas away from the river.
Des Moines Wikipedia, scroll down to density mapBusiness jobs exceed government jobs (this is why Minneapolis is bigger than St.Paul), and eastern Des Moines is chopped up by rail yards (similar to (north)eastern Minneapolis. Jobs with the railroad and associated industry will be lower paying than in the insurance business. The Des Moines River prevents expansion northward, pushing it more to the northwest. The Raccoon River blocks direct southward expansion, leaving a small area southeast of downtown and south of the Raccoon. The airport makes the area less desirable as a residential area.
Because of the angle of the Raccoon River, West Des Moines can expand slightly southward, and is more SSW (or maybe WbyS) of Des Moines. It appears that the I-35/I-80 north and west bypass facilitated westward expansion of West Des Moines and Urbandale, rather than triggering the expansion. You can build offices and commercial areas along an interstate, and provide jobs with commuting from the east and west. I-235 has facilitated commuting downtown.
There is a south and east loop, and an effort to have it redesignated as an interstate highway (it is claimed that if it shows as an interstate on GPS, it will make Des Moines look like a real metropolitan area, and also enhance sales of freeway frontage (apparently I-435 frontage is more sexy than IA-5 frontage). The loop south of the airport, should provide easier access to West Des Moines from I-35.
Des Moines has annexed south to the Warren County line. Microsoft is planning a major data center in West Des Moines south of the Raccoon River. The data center will be Microsoft's largest, and third in WDM. It is south of the Raccoon River on the Madison-Warren line, and just west of I-35. WDM is proposing to build an extension of the southern bypass west of I-35 (Veterans Parkway) which would connect to your bridge which is part of the Grand Prairie Parkway.
In 1950, Des Moines had 177K persons, and WDM was the only visible suburb with about 6K. Other towns, if they existed had 500-2K or so. Des Moines now has 215K, with most of the growth happening between 1950-60 and 2010-2016.
But during the post-WWII the suburbs to the west have exploded: West Des Moines 65K, Urbandale 43K, Johnston 21K, Waukee 20; Clive 18K, and Grimes 12K. Incidentally, the city limits are reminiscent of the suburbs in Johnson County, Kansas where the cities have expanded westward and southward to avoid being cut off from growth opportunity. It appears that West Des Moines has expanded southward to avoid Des Moines getting to the south of it.
Ankeny to the north, now has 59K. It has access direct access via I-35. It is physically separate from Des Moines. I don't know whether this is due to coal mines in the area with the possibility of tunnel collapses, or some other causes.
Altoona to the northeast 18K and Norwalk to the south 11K have suburban growth patterns, but this is much more recent than the western suburbs, so they are 40 to 50 years behind. Norwalk is an exception to no growth to the south.
Overall, Warren County has had strong growth. It surpassed its 1900 peak by 1960 and has continued to grow (about 20% since 2000). Indianola has about tripled since 1950. It has direct access to Des Moines via US 65 and 69, and its high school plays in the largest division (48 largest high schools in Iowa). I would consider this an exurb. Not directly adjacent to Des Moines, but commutable, and growth based on that.
Madison County might surpass its 1900 peak by 2030. Winterset is about twice as far as Indianola from Des Moines, and is considerable distance from the interstate.