by Michael Ronkin,
March 30, 2004
When discussing street width, there are 3 factors to consider: lane width, total street width, and number of travel lanes. In my opinion, the latter is the most important when it comes to walkability and pedestrian safety. The engineering discipline that determines lane widths (roadway engineering) is NOT the same as the one that determines the number of lanes (traffic engineering). The latter determines the number of lanes “needed to carry the anticipated traffic volumes,” and roadway engineers then assign lane width to each element to come up with a total street width.
Number of lanes matters for several reasons:
Total street width is far more dependent on the number of lanes than the width of each lane: one 12-foot lane widens a street as much as adding a foot to each of 12 lanes! Traffic speed is more dependent on number of lanes than lane width: recent research disappointed a lot of us when it concluded lane width had little effect on travel speeds. But an overbuilt street with capacity to spare allows drivers to go very fast at times when traffic is light – there’s a buffer next to them (an empty travel lane).
Pedestrian safety when crossing a street is exponentially proportionate to the number of lanes: it becomes increasingly complex to try to assess a safe gap in traffic when crossing 4 or more lanes of traffic in two directions. Also, traffic conditions can change in the 15-20 seconds it takes to get to the far side of a multi-lane street, as cars enter the traffic stream or accelerate. Pedestrians rarely get hit stepping off the curb – they get into trouble in the 3rd, 4th or 5th lane (or beyond).
So while we rail against AASHTO and its recommendations for lane widths (though [AASHTO] NEVER states 12 feet as a standard, and goes down to 9 feet), we should be focusing our energies on challenging the Highway Capacity Manual – the HCM guides traffic engineers through the analysis needed to determine number of lanes.
[Other safety hazards to beware of when dealing with traffic engineers and statements they commonly make:]
1. Traffic engineers tend to favor crash rates over absolute numbers. If traffic doubles on a highway, and crashes go up 50%, the rate has decreased.
2. Crash rates do NOT translate well across [forms of travel]: fatalities per 100 million miles is an absurd measure for walking.
3. At a conference in Switzerland years ago, I heard a presenter make the case that walking, bicycling and driving are just about equal [in terms of crash probability] on a PER TRIP basis, a measure we don’t use here. Walking trips are short with quite a bit of exposure, but using [the per trip] measure, stacks up very well against driving, covering long distances.
4. If Americans based their travel mode choice on safety, we’d all be riding busses and trains (public transportation comes out way on top), and we’d quickly abandon our private cars (just about the most dangerous [form] of transportation).
5. We talk safety, but do we walk it? At [a Transportation Research Board conference], I heard a fascinating presentation by a team of US transportation officials who took a European “study tour” to look at signalized intersections. They were surprised to hear the Europeans [unlike we Americans] meant it: they said they would sacrifice capacity and air [quality] to make an intersection safer. Europeans looked at absolute numbers, not rates. So if widening an intersection to add capacity led to more crashes, even if the crash rate went down, this was deemed unacceptable. I’ve lost many battles over pedestrian safety because the proposed solution slowed traffic too much.
6. In pedestrian and bicycle safety circles, we’re looking more and more at rates. A couple of recent studies are pointing in a direction we should all take heart in: pedestrian crashes are lower in areas of high pedestrian activity. Ped crash rates seem to correlate more with vehicular speeds and volumes (and street width) than pedestrian volumes. So getting more people to walk (and bicycle) is a way to increase safety. Which is contrary to the message NHTSA has been promoting till recently, which is “walk less and bike less.”
And now for some levity. Here’s a engineer joke I told recently to a room full of transportation engineers in Cheyenne and they loved it:
The Institute of Transportation Engineers is holding its annual convention in Cheyenne. The hotel is across the street from the convention center, so every morning they cross Main Street to get to their meeting. On the first day, an extraordinary event occurs, one that hadn’t been seen in over a hundred years: a herd of bison – 500 in all – stampede through town and a transportation engineer is killed. His peers lament his loss, and one of them proposes they put their collective minds together to come up with a solution. So the next day is spent in work sessions, and they devise a bison-proof pedestrian crossing. They build it overnight and it’s ready for use the next morning. Well, you guessed, against all statistical odds, another herd of bison comes stampeding through town, this time 1000 in all, and another engineer is killed crossing the street. Lamentation, wailing, grief, sorrow and guilt are expressed, till one optimist gets the crowd to quiet down and declares cheerfully: “Hey, we succeeded, we cut the rate of transportation engineers killed by bison in half!”
Bicycle and Pedestrian Program Manager
Oregon Department of Transportation
355 Capitol St NE 5th floor
Salem OR 97301