If you’ve ever driven over two cables strung across the road and wondered if you were part of some obscure science experiment or maybe caught in a speed trap sting operation – you’re half right. You and every other vehicle that encounters those cables will be counted, tallied, and divided by 365 in order to gather an overall volume measurement of vehicular traffic on a given roadway. Transportation professionals and industry experts refer to this number as Average Annual Daily Traffic or AADT. This number is part of the process where traffic volume is analyzed against traffic capacity influencing everything from land use and planning to speed limits and sight lines.
From the I-75 junction near the Pasco/Hillsborough county line to the downtown interchange, I-275 carries an average of roughly 142,000 AADT in both directions, varying by exit. Of those, two-thirds (94,000) are occurring locally within a six-mile stretch between the exits at Fowler Avenue and Columbus Drive. The remaining 48,000 trips are a combination of regional trips to and from Pasco County, vehicles entering I-275 north from the downtown interchange, or trips greater than six miles (i.e.: from Columbus to Bearss).
This amount of local traffic on I-275 is inherently incompatible with the regional purpose of having an interstate. Interstates limit access by design in the name of efficiency; having exits further apart means less points of conflict between vehicles already on the interstate and those entering or exiting. This can make sense in rural or suburban contexts where development is further apart, and space is readily available. For Tampa, I-275 averages an exit every mile, in the densest neighborhoods in the county, between the city’s two premier employment centers (Downtown Tampa and the University of South Florida). What would this corridor look like if we designed for those local trips first?
In addition to an overall measurement of traffic volume between major exits on the interstate, AADT is also counted at each point of connection. Entry and exit data is counted simply at each individual on or off ramp. So, for the “folded diamond” or “partial cloverleaf” exit at Busch Boulevard for example, this data gives us a sense of traffic direction. For I-275, the standard exit has four directions: 1) northbound on-ramp, 2) northbound off-ramp, 3) southbound on-ramp, and 4) southbound on-ramp. Using Busch Boulevard again, we can see that major commuter travel uses the northbound on-ramp at MLK Blvd to the northbound off-ramp at Busch Blvd and returns first via the southbound on-ramp and then finally the southbound off-ramp. A few exits are limited to only one or two of these directions and they do not currently track time of day – which is an important component – but does give us a sense of general patterns.
As stated before, most of the traffic volume on I-275 exists between Columbus Drive and Fowler Avenue – or approximately six miles. What the on and off data illustrates in this case is that the majority of traffic direction is following a commute pattern between Downtown Tampa, it’s urban neighborhoods and the three exits connecting the University of South Florida – one of the region’s top employment centers – at Fowler Avenue, Fletcher Avenue, and Bearss Avenue. Northbound traffic is significantly higher in the direction towards USF and southbound traffic returning on those same exits is equally disparaging. This commute pattern has largely been assumed to be true and has long been considered a target for future transit investment.
In order for any interstate removal in this area to be successful, significant investment is needed to reduce the vehicle miles traveled. The combination of concentrated employment at USF and a density of housing within proximity, provides ample possibility for transit ridership to begin that process for local commuters. Commuter alternatives, intelligent traffic monitoring, land use, and smart growth can begin to reduce the corridor overall.
AADT by Exit & Direction - Data from the Florida Department of Transportation
A traffic engineer once told me that removing I-275 was like squeezing a full water balloon: water would simply expand it until it popped. I laughed and replied that I wasn’t trying to squeeze the balloon – I’m trying to reduce the amount of water inside. In essence, this is the law of induced and reduced demand. Adding capacity to roadways only creates more demand. Inversely, removing capacity on roadways creates less demand.
A core philosophy of this project has always been about this law. It is unsustainable, expensive, and negligent to continue to accommodate interstate expansion in the name of aiding congestion. Cities like Atlanta, Los Angeles, and Houston have expanded their interstates multiple times and find themselves with even longer commutes. Tampa has an opportunity to end that trend and begin to reverse course. In addition to the reduction of congestion and the data-driven technicalities of this section, the moral and equitable arguments are just as compelling, and frankly, should be reason enough.