The Post’s Rachel Seigel takes us on an economic journey through the “urban doom loop” and explores this threat to midsize cities. Then, Teo Armus shows us a creative way we could try to avoid it. Read more:According to Columbia economics professor Stijn Van Nieuwerburgh, cities across the country could be heading for an "urban doom loop" that starts with vacant office spaces and spreads through downtowns. Later, Rachel Seigel joins The Post’s Teo Armus in Northern Virginia to experience a place that is creatively using vacant office space to escape the doom loop fate.