Diminishing returns of off-street bicycle facilities

This is a subsequent post (part 2 of 3) focusing on characteristics and perspectives of off-street facilities.

Some attention to my previous post seemed to stem from the incredulity of implying anything negative about the Midtown Greenway—one of America’s most beloved darlings of a bike path[1]. At the risk of appearing to cave, I agree with a majority of the comments. The Greenway raises the profile of “real” spending on non-motorized infrastructure. That funding, while high for bicycle facilities, still pales relative to auto-based infrastructure. The Greenway provides blissful riding which helps lure more cyclists. The Greenway catalyzes economic development. The list can go on. But there is room on the table for the sentiments mentioned in many of the comments (and subsequent posts) and those in the original post.

A main point is that while off-street networks are great, there is opportunity to evaluate some of their characteristics and costs amidst the larger transport systems. At the risk of fanning some flames from the original post, here is another perspective.

The on-set of off-street networks

Several factors influence the quality and extent of a city’s off-street bicycle network. Legacy effects of railroads have a lot to do with it. The perimeters of rivers and lakes, when they are available, are pretty easy to line; the founding fathers of city planning for Minneapolis leveraged this aspect to the max. Having cheap and available right-of-way space is often important. Access to a good chunk of money to spend—pork barrel or other—helps build these facilities where the natural landscape would otherwise preclude such.

The most progressive cycling communities continue to augment their off-street network: adding a new off-street bike path here; installing a grade-separated underpass there. Such facilities provide much value to the transportation network for cyclists. When strategically placed and effectively implemented by bicycle planners, they also go a long way to induce more users.

But off-street improvements are more costly than most of their on-street counterparts—an important consideration when expending public funds. At some point, building more of them fails to yield as much return. The outstanding question is when might that web be almost full?

A change in priorities in Minneapolis

Bike planning in Minneapolis reached a critical point a few years back with regard to their off-street network. The financial wells dried up and most space to expand was already spoken for. The city had already leveraged what it could by putting paths in railway and open space corridors; correspondingly, Minneapolis turned their focus to complementing their renowned off-street network with an outstanding on-street network. Other advanced bicycle cities are following suit. They are thinking more strategically about the costs and merits of adding more off-street facilities and this is a healthy thought process.

Has Minneapolis mostly caught the flies it can with its with its expansive off-street network? Many advocates contend that off-street paths are necessary to connect every nook and cranny of a city. Only then will the most timid rider come out of its shell. But, understanding the extent to which the “web is almost full” is analogous to the law of diminishing returns. Consider the 10k runner who successfully reduced her race time to under 45 minutes in six months of training but cannot break 44 min with even a year’s worth of training. Reflect on the morning delight that an initial cappuccino provides, which is less so for the second or third. The classic example from economics reminds us of the company who throws more workers into an assembly plant—which actually decreases production (e.g., the workers get in each other’s way, they end up waiting for parts). A cousin to this theory comes to us via Metcalfe’s Law, which is an inherent property in the design of a network—Ethernet, transportation or other[2]. The more extensive the network becomes, each incremental addition has a reduced overall impact. Can bicycle facility planning benefit from such thinking?

How to research diminishing returns

Ideally, this requires time use information. Gathering data about the onset of off-street paths; then doing the same for a dependent variable of your choosing[3]. Try to control for confounding explanations to the extent possible. Assuming the initial effects are largely linear, discern if and when the two trend lines diverge. Alternatively, gather a bunch of cross-sectional data and use regression modeling to uncover threshold effects.

But time use information is hard to come by. Reliable data about the nature of off-street facilities is similarly difficult to glean. These facilities are defined and measured differently; I can tell you that what an off-street facility means in Minneapolis is far cry from how the Italians define it. And, expansive off-street facilities sometime contribute little to a city’s useful cycling network for transportation purposes (as opposed to recreational purposes)[4].

Ranking of off-street networks

My inner bicycling sense suggests that Minneapolis has largely milked most of its potential from its off-street network; Boulder, Colorado even more. If we furthermore accept the turn of priorities in Minneapolis as an indication of such, what is that level and how does it rank? Minneapolis has 85 miles of off-street paths spread across 55 square miles of city [converting to kilometers, 136 km over 142 km2] for a density of 0.96. Other top cycling college towns of Davis (California), Boulder (Colorado) and Madison (Wisconsin) come in at 3.2, 1.3, and 0.34, respectively. Big city comparisons are 0.65 for Washington, DC and 0.35 for Portland (Oregon) [5], [6]. Minneapolis is among tops in the list, notably so for a relatively large city (see below, values represent: Off-street facilities (km) / Area of city (km2))

1. Davis, California – 3.2
2. Boulder, Colorado – 1.3
3. Minneapolis, Minnesota – 0.96
4. Washington, DC – 0.65
5. Portland, Oregon – 0.35
6. Madison, Wisconsin – 0.34

Other considerations in thresholds

Fully thinking about a point of diminishing returns in each city requires us to consider local needs and local opportunities[7]. Much of the network in Davis was built as part of new development. Boulder has lowered the pains of building their network by leveraging space adjacent to creeks and the sort and also building requirements into new development. And even if a threshold might be exceeded for bicycling, that does not necessarily mean that more of it would not benefit other users (e.g., pedestrians) with different needs. But the above analysis does suggest that an abundance of off-street facilities is not necessarily needed to obtain status as a well respected cycling city, at least for U.S. standards.

There is always more to the story. The role of discontinuities of the off-street system plays an important role. And, the degree to which an off-street system is complemented with a solid on-street networks is paramount. The aspiration of many advocates—a complete off-street cycling network to every address in a city—is out of reach for almost all communities. It is helpful to consider a point of diminishing returns, given that off-street facilities cost more than their on-street counterparts. That point will inevitably vary by community and its local characteristics. In Minneapolis, the density hovers around a “single kilometer of off-street facility per square kilometer of city size” –a convenient threshold to help start such discussions.

The next post (part 3 of 3 addressing off-street facilities) will be a lighter fare.  After all, it is scheduled for the day after Thanksgiving; it will focus on Bologna’s (Italy) network.