There are no formal statistics kept on Chicago parking lot transactions, but developers and commercial real estate brokers say the market for development seems especially active these days. The value of parking lots just east of the expressway downtown, measured by the number of square feet that can be built on a site, has increased 25 percent in the past two years, estimates Mike Nardini, a CBRE first vice president. Owners of lots continue exploring potential sales — especially those who missed out on the last real estate peak a decade ago.