Urban economists have long known that urban patterns emerge in response to individual residents and businesses optimizing the time it takes to connect. One optimizing move (a family moving into a far-flung neighborhood) creates another move (a grocery store moving into the neighborhood because the new family is enough to create a critical mass of customers) that in turn prompts more moves. This ongoing optimization act is guided not by a centralized decision-making mechanism, but rather by relatively simple rules—most notably, the need to make daily connections within a limited amount of travel time, an hour on average. Consistent urban patterns form. In large metropolitan regions, a “circus tent” of urban form emerges from the balancing act, with downtowns at the center and the first ring of three to four “edge cities” around 6 to 9 miles from downtown. Metropolitan areas have grown beyond the tent to form recognizable megaregions stretching along high-speed transportation corridors. A hierarchy of job-oriented centers is spaced in regular intervals along the megaregion.
Cities are technological engines using proximity and speed to optimize connectivity. The balance of either strategy varies across urban landscapes and regions. The density of development in city centers shortens trip distances to enable relatively slow travel modes, such as walking, biking, and slow speed car and transit. High speed roads and transit move people from far away suburbs into major jobs centers.
This concept of travel time is a powerful measure that illuminates and quantifies the way development happens. It is the key factor in market studies prepared for private developers, retailers, and others when determining when and where the next opportunity lies.
City planners continually seek ways to create great communities—places with a diversity of opportunities for people to work, play, socialize, recreate, etc. that can be accessed in a reasonable travel time. Tensions start to emerge when travel times stretch far beyond an hour a day. As region’s become more dispersed and suburbanized, speed becomes increasingly important and congestion is the key measure of transportation system design. Ultimately, longer suburban trips overwhelm roadway network capacity resulting in diminishing returns. They also create negative impacts to air quality and promote sedentary lifestyles.
Attention is shifting back towards increasing development density so that proximity carries more of the connectivity load. But how and where does that density occur? Can the relatively small geographic footprint of exiting city centers move the needle, or will we need to densify the suburbs? Research has demonstrated that traditional neighborhood development (TND), touted as a way of densifying the suburbs, has not noticeably shifted the need for speed. Transit oriented development (TOD) creates density along premium transit corridors and is moving the needle, but it requires significant investments and supporting land use plans and policies. Can new cities, in vogue in the 1960s, with dense downtowns and premium transit corridors, appreciably shift us towards connectivity through proximity?
All of those weighty questions can be answered using the simple rule that guides city emergence; optimized travel times to frequently accessed destinations. The rule has been and continues to be used to measure success, unfortunately it focuses on level of service, with a focus on maintain speed, because most land development over the past 80 years has been in the suburbs. Level of service has its own narrative that plays out in front page headlines over the negative impacts of density induced congestion. Seldom do those headlines note that density brings destinations closer, thereby reducing the need for speed.
A new set of travel time-based measures is needed. Multimodal accessibility (MMA) quantifies the number of destinations reachable to any given location by any and all modes. It provides insight into where opportunities and tension exist. Multimodal system productivity (MSP) quantifies the productivity of connections to provide more detailed feedback on opportunities and tensions. Both measures are agnostic regarding proximity or speed, change in one or the other, or both, will influence the results of both. Because they are agnostic, they provide a common analytical platform and language for land use and transportation planners.
While both measures are new to the planning profession, they rely on theoretical constructs that have been used by market researchers and regional travel demand models for years. Renaissance is on the forefront of developing, refining, and applying the measures. We believe that the MMA and MSP create a much needed and insightful planning framework for the 21st century that can help planners answer a wide range of questions.