字幕表 動画を再生する 英語字幕をプリント This road might not look like anything special. But what if I showed you the same road, a few years earlier —notice anything different? Here's a better angle of that: The old road has 4 lanes for traffic. The new one has two. And now there's this middle lane for left hand turns. There's also a new bike lane. This is what transportation planners call a road diet. And it's a very popular to make the roads safer. Over the course of the 20th century, four lane roads became an American institution. It started with the release of Ford's model-T in 1908. A few decades later, there was one car for every two households in the states. By the 1960s, many roads became so busy, that traffic engineers had to figure out how to add capacity. So, they added lanes. A lot of pavement was getting put on the ground. Even where maybe when the population and traffic volumes weren't so high that we needed that pavement. But we wanted that pavement. Sometimes your eyes are bigger than your stomach. Fast forward to the present day and we're left some overbuilt roads that are pretty unsafe. And crash reduction is a major benefit that planners can achieve with just a bit a paint. Four lane roads have quite a few conflict points -- these are places where accidents could happen. These ones represent merging accidents. These are rear ends, and there are multiple left turn crash scenarios. Now look at what a 3 lane reconfiguration does — There are far fewer crash points. A road diet can also make left turns suck way less. The shared middle lane takes left turners out of the traffic flow, so they won't hold up drivers who want to continue through. And now, left turners will only have to cross 1 lane of traffic instead of 2, which will eliminate broadside accidents. And the benefits don't end there. By slimming each lane in the road, the road diet reduced the travel speed by almost 7 miles per hour. Narrower lanes can cause that psychological impact on the driver to slow down a bit. And while a 6 mile an hour difference in speed may seem modest, it can make an auto accident much less deadly ... Narrower lanes also leave more space for expanded sidewalks or bike lanes. your pedestrians will feel safer. It might've given you more green space to separate from your vehicles. And your bicyclists might have a dedicated space to ride. In the midst of these changes, the number of traffic lanes has gone from 2 to 1. So if you drive a car, you might assume that the tradeoff of a road diet would be congestion... How could traffic *not* increase? That's usually their concern before a road diet is implemented. That's not what happens. The volume of the roadway is still sustained. We wouldn't want to put a 4 to 3 conversion on a piece of roadway that would then push half your traffic somewhere else. And what we found in a couple of places, it actually makes moving through town easier. But traffic flow is only one part of the equation — you've got to also balance commercial and safety benefits too. especially if this is through an urban corridor where you've got businesses, coffeeshops, it works. it keeps people moving, it doesn't take traffic away from that corridor, and it reduces those rear-ends that thendo stop the road. Iowa is conducting this road survey because so far, the plan has worked *really* well for them. A study of 15 streets in the state saw a crash reduction rate of nearly 50%. At the same time, the diet didn't substantially disrupt other activities along their corridors. A key factor was the traffic volume, measured by engineers as 'Annual Average Daily Traffic'. Most road diets will run into problems as you approach that 15,000 vehicles number. In Iowa, many roads don't get that much traffic. The same could not be said for the suburban contexts in California. A study comparing road diets in Iowa to corridors in California showed that California's streets average about double the amount of daily traffic. The same kinds of road diets resulted in a 17% crash rate reduction -- significantly lower than the reductions in Iowa. This is not to say that the road diets in California are an outright failure. It's just a difference *context* for the road diet. And the success of a road diet is driven by so many other factors -- economic impact, land use, or level of service to name a few. What works in Iowa may not be the right fit for California, and vice versa. So the case for road diets is pretty clear: they do slow streets down and they do reduce crashes. But whether or not that's worth the trouble depends very much on the context of the world that surrounds the road.