Executive Summary
Despite today’s horrible traffic congestion, it is tough gaining support for expanded road capacity. Amassing the funds for major urban expressway projects is slow under centrally allocated trust funding because of the political pressure to spread annual appropriations over most districts and different modes. Many planners and transit advocates are quietly happy with the growing misery of road congestion. They believe public policy should drive people to transit use, or to higher density living near their jobs. Others believe that no matter how much road capacity we add, it will simply fill up to congested levels. But perhaps the most widespread belief is that there is simply no room left to expand urban highways without enormous negative consequences.
Compounding the problem is that people don’t like the look or feel of many of our big highways. They have gotten so large and so bleak that they are offensive, like some kind of alien implant in our urban areas. A dislike of highways predisposes people to dislike all proposed new road projects, even those that are designed with more concern for aesthetics and better mitigation of impacts.
This study argues that in too many American cities, despite much planning, we are planning highways poorly. Traffic generation studies show the optimum expressway network is on a grid of roughly four miles with a denser grid of surface arterials with signalized intersections at intervals of between half a mile and three-quarters of a mile. Instead of planning such a denser grid of modestly sized roads and innovating with new kinds of roadways, we have simply enlarged the existing too-sparse grid of highways to gargantuan proportions.
This paper suggests new ways of thinking about highway design. Many of our highways have gotten too big, not because anyone wanted them to be that way, but because widening an existing highway was the simplest thing to do at each point-the line of least political resistance. But ever-wider highways create cascading functional problems. Multilane ramps are needed at interchanges of expressways, while on surface arterials enormous intersections with multiple turn waiting lanes are needed, plus long signal cycles. Overly wide arterials make handling left turns a huge challenge, and in the end limit capacity.
But with the kinds of innovative design concepts discussed in this paper, highways needn’t get ever wider. They can be built upward as elevated roadways in certain contexts, using new, cleaner styles of construction. Conventional double decking involves enormous structures, but car-truck separation can make possible far smaller scale double-deck structures for auto-size vehicles only. Creating specialized roadways is one key to a more sensible highway system. Heavy truck volumes can be handled by specialized truck-only roadways. And in some places delivery and pickup, and other high-value commercial trips that will pay for premium service, should be catered to separately.
We need to find ways to build better grids of highways-smaller highways but more of them. At the expressway level denser grids could keep most expressways to six lanes or less, while on surface arterials spaced at half a mile, left turns become manageable without resorting to acres of asphalt at intersections and two-minute signal phases.
One key to adding more highway corridors is to make use of underutilized railroad rights of way, power line reservations, and flood control channels. These need not be several hundred feet wide to be useful; a four-lane, value-priced congestion-relief roadway for cars and buses needs as little as 60 to 70 feet. New designs handle the left-turn traffic at stressed arterial intersections, using limited grade separations.
The biggest move to make highways more acceptable is likely to be moving some of them underground. A lot of this is already happening overseas in Europe and Australia where urban highways were less developed than in the United States until recently. Where we have need for increased capacity in bottleneck corridors we’ll need to look at undergrounding- either beneath the existing corridor or parallel to it several miles away-because no surface facility or elevated structure is acceptable. Undergrounding ranges from entrenching within walls, to caps, cut-and-cover, and more extensive mined tunnels or those built with tunnel- boring machines.
Adding capacity with innovative design concepts is generally more expensive than adding lanes to mammoth freeways. But congestion and loss of mobility from not providing needed highway capacity are also hugely costly. Our productivity and quality of life depend heavily on being able to move ourselves and freight swiftly and predictably around our metropolitan areas. That way people have a wide range of opportunities for jobs, shopping, education and recreation, and employers have the widest choices to hire labor and services and get supplies and shipments handled efficiently. Areas that provide good internal mobility will thrive and prosper while others will languish. Innovative design will be essential to gaining acceptance of needed additions to highway capacity.