Editor’s Note: One of the missing voices in bicycle planning in the Twin Cities is college students who belong to a generation much less likely than their parents to own vehicles. This series of posts written by Macalester students for the “Bicycling the Urban Landscape” course are one effort to include these voices. This piece was contributed by David Munkvold, a senior at Macalester College who bikes everywhere. If not on his bicycle, he might be slack-lining, programming, playing music, or meditating. David is very interested in the infrastructure of the Twin Cities as he will continue to live here after graduation.
Coming from a medium-sized Midwest town with little bike infrastructure, I have always viewed urban cycling as a kind of puzzle. There is a start point and an end point, and finding the sequence of surfaces that allows me to reach my end point without any bodily injuries is the challenge. After moving to St. Paul to attend college, I was initially relieved of this perspective, as the dense bike network of the Twin Cities made me feel as though my routes were no longer problems to be solved, but yellow brick roads leading to my destination . However, after biking year-round here for four years, I have come to realize that as a cyclist, there are always struggles to find appropriate surfaces to traverse upon. Where at home it was a lack of infrastructure, here it is the ice, snow, and rampant road decay that present obstacles to safely riding in the city.
There are many instances in which our wonderful bike paths are made obsolete by our latitudinal condition. Ice and snow that cover the roads make it impossible to bike without certain tires. Plowing of the streets results in pile-ups of snow in bike lanes or parking lanes, and residents aren’t afraid of encroaching on the bike lane when there isn’t enough room for their vehicle. For example, on Marshall Avenue in St. Paul, there are stretches of road where snow piles block the parking lane and motorists park in the bike lane. There are few streets with bike lanes that are wide enough to hold plowed snow, bikers, and motorists without overlap. When the snow and ice melt away, they leave deep puddles and cracked concrete in their wake. Even in the middle of the summer, the erosion of the road surfaces can make it difficult to bike on many streets and paths. On the western end of Summit Avenue in St. Paul, which was resurfaced about five years ago, the erosion has produced street-wide cracks that occur at regular distances, making it taxing for cyclists who must endure a rhythmic test of skeletal integrity.
Many are content to deal with these challenges because they seem to be unavoidable truths of living in a colder climate. While we cannot do anything about the cold air, the snow, ice, or sleet, we can do something about the surfaces that withstand (or fail to withstand) these features of harsh northern winters.
A road surface’s biggest threat in colder climates is moisture. Most road surfaces are relatively water-proof, and we expect that the moisture will either run off into the gutter or evaporate. Due to the material properties of asphalt and concrete, however, water inevitably finds its way into the road surface and becomes trapped. During the winter, this water freezes and its volume increases by around 9% of its original volume. This expansion creates tension in the road surface and eventually breaks it up from the inside. Not only does the water inside the road surface freeze, but any water that was on top of the road also freezes, creating dangerous conditions for drivers and bikers alike. The freezing of moisture inside and on top of the road during the winter make it less safe year round, either through ice/snow or deteriorating street surfaces. How can we keep moisture away from the road so that we can avoid dangerous transportation conditions?
Rather than finding ways to keep moisture away from road surfaces, St. Paul has decided to implement a bike path downtown with a surface that actually soaks up moisture. Pervious concrete, the idea for which has been around for centuries, allows water to pass through it into a reservoir underground. Water doesn’t accumulate on top of pervious concrete, so cyclists won’t have to ride over ice and snow. Water doesn’t accumulate within the concrete either, so volume expansion due to freezing is not necessarily an issue. It’s too early to tell how effective the surface is, but if it is at least as durable as the materials we use now, then we should begin using them across the city, and not just for bike lanes.
Hypothetically, if our roads soaked up water, we wouldn’t need extra space for storing snow. Provided that the reservoir was built deep enough to avoid saturation of the concrete, then the snow should be able to pass through it and into a network of runoff that can be managed more easily than runoff from waterproof concrete. Less snow storage means more room to drive, bike, or walk and safer conditions overall during the winter months. It also means that there is less moisture trapped in the road surface that could freeze and damage the concrete. Not only is pervious concrete potentially more durable than concrete, but it can be cheaper to construct. Conventional concrete and asphalt roads require the construction of above and below ground storm water retention ponds and gutters. Pervious roads require smaller capacity storm sewers that only need to be underground.
Although pervious concrete is cheaper to install, it does require maintenance. Debris must be regularly vacuumed out of the concrete to prevent clogging of the porous material, or else water will become trapped inside and the road will be subject to cracking and buckling. Considering that the city would not have to devote as many resources to plowing the streets during the winter and repairing the streets during the summer, it is a small price to pay.
It will be crucial to keep an eye on the pilot pervious path in downtown St. Paul for the next few years and be prepared to make some changes to the way that we think about surfaces in an urban environment.
Sources
Montgomery, David. “How Does Minnesota Maintain Its Roads?” Twin Cities. Twin Cities, 18 Apr. 2016.
Washington State Department of Transportation. WSDOT STRATEGIES REGARDING PRESERVATION OF THE STATE ROAD NETWORK.
As an ancient Mac alum (’73) myself, it’s always fun to read “Macalester Student Perspectives”. One comment I’d make about vehicle ownership is I think today’s students are much more likely to own a car than in my day. In the early ’70’s it was rare for a student to own a car. The parking along DuPre Hall, for example, was half empty back then, while today I think it would be a struggle to find a space.
There were a lot of bicycles, perhaps as many as today, and students relied heavily on buses, walking, and hitchhiking.
Pervious concrete sounds like to good surface for bike trails. Thanks for the article.
Good points, really like the description of the local challenges of winter (and summer) biking.
Since I’m a civil engineer, I want to flesh out some of the discussion of pervious concrete. It isn’t necessary more freeze/thaw durable than regular concrete as you seem to imply.
(This is part where eyes might glaze over, please ignore if don’t want technical) Big spaces in concrete that pervious concrete provide don’t take care of the freeze thaw expansion problem, its the water in the cement past around the aggregate that breaks up the set/hardened cement that is most of problem – but aggregates can suck in moisture also and break up in f/t cycles, (hence MN DOT having approved aggregate pits). There is a whole science around the proper size and ratio of air bubbles in concrete needed so f/t expansion doesn’t harm the cement past, and proper size is tiny. Some nerds have gotten into this so much they now think the worse damage occurs when the frozen water in concrete starts to warm up and expand, just before it melts, while MN DOT would say its not the cycles but long cold freezes that do the most damage. Regardless, big voids in pervious concrete don’t help much with this, and in some ways be even worse, voids will spread the salt-filled water everywhere, quicker, than denser concrete. (most f/t damage is connected to saline water). Dense concrete is still pervious but water spreads through it slower.
Also, pavements aren’t that thick, so even with pervious concrete that is much more open area than dense concrete, the pervious pavement itself can’t hold much water before it is filled. Pervious concrete, or pervious pavers, are usually designed with much more extensive open aggregate below the pavement, to hold water until it can seep into soils below.
So pervious concrete with extra aggregate under it, can be quite a bit more expensive to install than regular concrete with a few inches of crushed aggregate under it. It can save money otherwise though – as the extra capacity to hold water on site, reduces needed stormwater pipes, detention ponds etc.
And posted was smart to bring up issue of cleaning pervious pavement, this needs to be done, but from real world applications I have seen, its not nearly the problem in most situations people make it out to be. And vacuum trucks are gaining acceptance as good alternative to street sweeping generally.
Having said all that, the proper drainage of road base materials is killer way to extend life of roads and we still have much to learn about the most economical way to build roads, so they cost the least on an annualized basis, if we figure in long-term maintenance issues. Putting more drainage in up front can greatly improve road performance, but of course, that involves politicians paying more now, to save money in future. Not the easiest thing to achieve.
Bike lanes with pervious concrete seem like a great way to experiment with this option and learn. It seems it would provide drainage zone for the main road, built old-fashioned way and possibly improve its performance also. Very interested to see how this works.
Hi Karen, The Netherlands and some other northern European countries have been shifting to using more and more pavers rather than cement or asphalt. They’ve told me that there are two benefits in addition to aesthetics; they allow more water to pass through faster which both reduces problems of water build up and reduces f/t problems, and repair when there are f/t problems is easier, faster, and less expensive since they can quickly remove damaged pavers, repair the base, and re-lay pavers and often without any machinery.
Also, I don’t know that they ever use dry salt. I believe they always use a brine solution.
One down side is that while they produce a surface for bicycling that is better than most of our MN roads it is still not as good as a proper asphalt surface.
Is that a viable option for MN?
Very interesting, both the article and Karen’s insights. I also wonder about the pro and con of pervious concrete when winter salt gets applied (as it surely would). Is dehydration worse, and deeper? Any thoughts, Karen?
Another way to make roads longer lasting is to make the underlying roadbeds more stable and the surfaces more uniformly level, as the Germans tend to do, which reduces the heavy slapping effects of traffic, especially from trucks.