This article is part of a series about the past, present, and future of commuting in America.

For six months a year, average daily highs in Copenhagen fail to crack 50°F and the sky is cloudy most of the time. It's not necessarily weather that makes you want to head outside and jump on your bike.

In San Diego, meanwhile, average daily highs tend to hover between 60°F and 80°F, and the sun shines 68 percent of daylight hours.

And yet Copenhagen has one of the highest rates of bicycle commuting of any city on the planet, with 37 percent of its residents commuting by bike. In sunny San Diego, meanwhile, less than 1 percent commute by bike.

American cities have been designed to make driving fast, cheap, and safe

The majority of US states and cities say they want to increase the number of people biking and walking, to reduce traffic, promote public health, and curb greenhouse gas emissions. The comparison between Copenhagen and San Diego shows how pivotal city design is in making this happen.

"People are rational. When they drive to work on a big six-lane road and see some guy in spandex pedaling with cars flying by him, they think, 'There's no way in hell I want to do that,'" says Andy Clarke, president of the League of American Bicyclists. "If you give people places to ride or walk where they feel safe, then they start riding and walking in greater numbers."

For decades, planners have designed American cities, towns, and suburbs with the primary aim of making driving fast, cheap, and safe. The result of that policy is that more than 85 percent of us drive to work every day — while less than 4 percent bike or walk.

To change this equation, cities and towns will have to transform their streets to make nonmotorized travel safer and easier. These are some of the most effective ways to make that happen.

1) Stop building cul-de-sacs and bring back the grid

One dominant factor is basically impossible to change once a neighborhood has been built: the shape of its streets.

Gridded streets let pedestrians or cyclists travel the same distance and reach a much greater number of destinations than cul-de-sacs or other sorts of street designs with fewer interconnections and intersections.

This is because people on foot can travel much in straighter lines to get to their destination:

For people in cars, driving slightly shorter distances is less important, and less interconnected designs with fewer intersections to slow them down can actually be much faster — which is part of the reason so many cities and suburbs that were built during the 20th century lack grids.

But research shows that as these newer designs aid driving, they discourage walking and biking. One 2009 study compared 24 different California towns and cities and found that people were more likely to walk or bike in more heavily gridded areas with more intersections and fewer dead ends. "We've deliberately built cities of cul-de-sacs and neighborhoods that are isolated from each other. That really makes it hard to bike or walk," Clarke says.

2) Change zoning rules to allow for density and mixed-use

Regardless of street design, cities can dramatically alter an area's walkability just by making changes to their zoning requirements.

Many municipalities have zoning laws that prevent businesses from operating in certain residential areas, as well as other sorts of restrictions that limit the number of residences that can be built in a given area.

But lots of research shows an even stronger link between these factors and the odds that a person will bike or walk to get somewhere. A study of San Francisco neighborhoods, for instance, found that those with higher levels of density and mixed uses saw much greater numbers of people using nonmotorized transport.

This isn't exactly a surprise: when you think of a walkable area, it's probably a relatively dense city neighborhood where you can walk to stores, shops, and other sorts of amenities. But for a city, changing zoning laws is much, much cheaper than physically altering road designs — and in the long run, it would likely make these cities money.

3) Eliminate parking requirements

Many cities provide lots of free on-street parking, or require that all new residences be constructed with off-street garages or lots. The idea behind the latter requirement is that if builders didn't do this, residents' parked cars would clog up street parking and leave a shortage of spots.

But parking spots cost money — to pay for land, pavement, street cleaning, and other factors. So the net effect of all this is to subsidize the cost of parking with tax dollars or rent money, regardless of whether a person drives. "This means people who don't own cars pay for other people's parking," economist Donald Shoup told me last year. "Every time you walk somewhere, or ride a bike, or take a bus, you're getting shafted."

The free parking subsidy is worth as much as $127 billion annually

He's estimated the annual free parking subsidy to cars to be as much as $127 billion nationally. For daily commuters who park for free, this subsidy can be worth more than the cost of driving, on a per-mile basis. This incentive makes people more likely to drive: one study found that people who live in residences in Brooklyn, Queens, and the Bronx with minimum parking requirements are significantly more likely to drive to work in Manhattan, compared with others who live and work in the same areas.

Additionally, if you want a dense, mixed-use city, mandating off-street lots and garages for all new residences is one of the last things you should do. Instead of letting developers and market demand dictate the number of parking spots for every new house and apartment building, it can lead to a glut of underused parking that makes neighborhoods more spread out and less walkable.

4) Put roads on a diet and make lanes narrower

In some areas that are dense and mixed enough to make walking or biking appealing, people still often opt to drive because of the busy arterial roads they'd have to cross. "That one big barrier can be extremely daunting," says Clarke. "It's often enough to put a lot of people off trying it."

One increasingly popular strategy to solve this is the road diet, which involves modifying a road so cars can't travel as fast. Most often, a second traffic lane will be converted into a turning lane, bike lanes, or a parking lane:

These changes can help bikers or pedestrians by giving them more space that used to go to cars, but the real goal is increasing safety for all parties involved by slowing down traffic. Studies have shown that these diets can cut down on crashes by anywhere from 18 to 25 percent.

In some cases, road diets can increase congestion. But it happens less often than you think, because turning lanes help cars get out of the way of traffic when they're waiting to turn. Research indicates that on roads with up to around 20,000 vehicles per day, these diets don't significantly reduce capacity or increase congestion. In some places, like New York, they've actually sped up traffic slightly by getting turning cars out of the way.

A related concept is lane narrowing. Starting in the 1970s, engineers began been building wider lanes — generally 12 feet wide — in most heavily trafficked areas because of the idea that it was more "forgiving" for drivers, allowing for a greater degree of lane drift and other errors without causing a crash. But lots of recent evidence has shown these wider lanes tend to make people drive faster, regardless of the speed limit, leading to more crashes and deaths for drivers and pedestrians.

As a result, some planners are calling for lanes to be reduced back down to 10 feet, the width they were originally built in the downtowns of many older US cities. A handful of municipalities have experimented in lane narrowing, and have seen fewer crashes — without reducing road capacity.

5) Build protected bike lanes

Bike lanes blocked off by some sort of physical barrier (like a curb or a row of bollards), rather than just a stripe of paint, started popping up in Europe in the 1970s. Recently they've come to the US, quadrupling in number since 2010.

They're a bit more expensive to build and take up more space than bike lanes demarcated by paint, but research consistently shows that people — especially those who are new to biking — are much more comfortable riding in them.

A 2014 study of eight new protected bike lanes in Austin, Chicago, Portland, San Francisco, and Washington, DC, found they increased bike traffic by anywhere from 21 to 171 percent on those roads, and that 10 percent of the cyclists wouldn't have been traveling by bike if the lanes didn't exist.

6) Connect bike lanes to create usable routes

The Washington Post recently created a great series of maps that show how bike lanes in most major US cities are a disconnected mess, forcing bikers to mix in with traffic on the majority of journeys.

Martha Roskowski of People for Bikes has compared this to a ski resort at which, to get down the mountain, every skier had to take ultra-difficult black diamond runs at one point or another. It doesn't matter if they can take easier green circles and blue squares to start out if they have to surmount a mogul-filled black diamond to get to the bottom.

In the same way, the League of American Bicyclists' Clarke says, connecting protected lanes and paths could go a huge way toward making new bikers feel comfortable cycling in a city. One obstacle is that in many cities, the bikes lanes already built were relatively low-hanging fruit: they were built on roads with extra lanes and low traffic volume, where they didn't significantly cut down on car capacity. In those places, the next steps are likely to encounter more opposition from drivers.

Why all this might be easier than it seems

The good news is that there's a strong correlation between the number of people walking or biking in a city, and the overall fatality rate for pedestrians and cyclists:

We don't know for sure, but many researchers believe there's causation moving in both directions here. More people walk and bike in cities where it's safer to begin with. But getting more people walking and biking might also serve to make those activities less dangerous.

We can see evidence for this second idea — often called "safety in numbers" — in finer-grained studies that track collision rates within different areas of a city. One Oakland study compared intersections with similar traffic volumes and speeds and found that at those with more pedestrians crossing, each individual pedestrian was less likely to be hit. Other studies have come to similar findings for cyclists. One explanation would be that at these intersections, drivers get used to seeing pedestrians and bikers, and watch out more carefully for them.

It's still not entirely proven, but if the "safety in numbers" idea is correct, it's good news for cities interested in promoting biking and walking. It'd mean that efforts poured in to making their neighborhoods walkable and bikeable can snowball — and the modest increase we've recently seen in people biking and walking could accelerate in the future.

Further reading: How Amsterdam became the bicycle capital of the world