Local residents have heard and accepted this story as gospel for a long time: Johnson County is the fastest growing part of the Kansas City area.

New residents and businesses have been pouring into the county since the 1960s and will continue flocking there for decades to come.

So let’s answer a few questions today, with the help of population estimates by the Mid-America Regional Council from late 2011.

• Which county will be the region’s largest in 2035?

Surprise, it’s Jackson County, not Johnson County.

• Which side of the state line will gain the most people by 2040?

Nope, not Kansas. Try Missouri.

• Which counties will have the fastest rates of growth between now and 2040?

It will be Platte County followed by Clay and Cass counties in Missouri. (Johnson County is only fourth highest.)

Summed up, for all the hoopla over the continued growth in Johnson County, MARC estimates show that the population of this area will tilt heavily toward the Missouri side for many more years.

The MARC figures should make it clear to the area’s elected officials, civic leaders and others that it continues to make great sense to invest in Missouri-side amenities, such as better roads, transit, schools and tourist attractions.

Here are some statistics that back up those conclusions.

According to MARC’s 2011 estimates, it now appears Jackson County will continue to be larger than Johnson County for almost another quarter century, or until 2036, when both counties will be approaching the 800,000 mark.

Johnson, Wyandotte and Leavenworth counties are predicted to jump by 330,000 people between 2010 and 2040. That’s a hefty figure, led by the region-leading total of 282,000 new Johnson Countians. However, the counties of Jackson, Clay, Platte and Cass are expected to grow by 414,000 people by 2040 (led by 156,000 more Clay Countians). Almost 60 percent of the metro’s population will live on the Missouri side in 2040, about the same as today.

Finally, the Northland can lay claim to being a true population juggernaut in this region. MARC estimates show that Platte County’s population could jump by 76 percent between 2010 and 2040, while Clay County could soar 70 percent. Cass County’s expected growth rate is 55 percent, Johnson County’s is 52 percent and Jackson County’s is 20 percent.

Of course, these are all just predictions. The MARC predictions could be wrong, and things could change dramatically.

Maybe the new Kansas income tax reductions really will lure tens of thousands more people over the state line. Or maybe the state’s economy will implode. Crime rates and school quality will be closely watched. These and many other factors will be at play in the coming years.

It’s also worth noting that long-range planning isn’t a strength of many elected officials in the Kansas City area, although there are exceptions.

Mayor Kay Barnes changed the face of downtown forever by pushing construction of the Sprint Center along with the Power & Light District in the early 2000s.

Currently, Jackson County Executive Mike Sanders and other supporters of commuter rail are working on a plan that could move people and woo economic development. Any decision this year on commuter rail — accepting or rejecting it — could affect population patterns for decades.

In Kansas City, Mayor Sly James and other ardent backers of the city need to be looking north of the Missouri River more than ever, making sure that roads, parks and other civic assets are being constructed, and that effective public safety is provided.

Meanwhile, school districts should be preparing for more children living in the Northland as well as in Lee’s Summit, eastern Independence and eastern Jackson County cities like Grain Valley.

Johnson County remains hugely important to this region’s future. But as the MARC estimates show, people will continue to live, work and play in a lot more parts of this community.