Cycletracks and the 85th Percentile

All modern transportation design guides addressing urban transportation, in an attempt to protect all persons of “all ages and abilities” while maintaining “flexibility”, require the use of cycletracks where there is an arterial or “connector” and the speed of the vehicles is over 30 miles per hour. That’s a lot to unpack. What we are really saying is that when you have car volumes as you see in most major urban locales for connectors, and those cars are moving at over 30 mph, it is critical that bicycle users on such roads be physically protected from automobiles.

Cycle tracks are, in gross terms, bicycle lanes that are protected from motorized traffic by physical barriers. The barriers could be lanterns, bollards, curbs (continuous or or not) etc. Painted “buffers” are not cycletracks, although many design apologists argue they provide “protection”, hence the rise of so much confusion as to what anyone might mean by “protected bike lanes”. The best practice is to use the terminology found in the design guide you reference. Hence the use here of cycletrack, which is the term employed by NACTO.

Confusion is further engendered because design apologists have suggested that if a road is posted 30 mph, it needs no cycletrack. Those folk have missed the “fine print” as it were, in that even the FHWA points out that the speed relied on in addressing the appropriate infrastructure is operating, NOT posted speed. So what do they mean by operating speed. In most cases they mean 85th percentile speed of all traffic. The 85th percentile marks the speed at which 85% of the traffic is doing that speed or slower. By way of example. If you have 100 motorists on a street, and 85 of them went 40 mph, 10 of them went 45 mph, and 5 of them went 50 mph, the 85th percentile would be 40 mph. Let’s assume that this street is posted at 30 mph. We can then see that 100% of the traffic exceeds the speed limit. 85% of the traffic traveled at 133% of the speed limit, and the 85th percentile is at 133% of the speed limit.

The fact is that most people do NOT travel at or below the speed limit. Indeed, it is rare that you don;t find someone traveling at least 5 mph over, in no small part because everyone figures that no officer is going to give you a ticket for going 5 mph over the speed limit. Historically traffic engineers RAISED speed limits to match the 85th percentile, and, as a result, the speed limit on Northern Lights was raised from 40 to 45 (though we saw numerous cases of people losing control of their vehicles at 40 mph). Soon thereafter the speed of traffic on Northern Lights rose, and now we see traffic moving along Northern lights at 60 mph (133% of the posted speed limit).

Is that 133% some magic number? Is the 85 percentile speed always going to be the speed of 85% of the vehicles? No. In quite a few cases where the posted speed is 30 mph we see speeds of 150%, and typically the less traffic the fewer cars are going to have synchronous speeds. But it seems that most people have no problem doing 45 on a connector, or 60 on a street to be divided highway like Northern Lights. It’s typically a matter of the driver’s perception of what is appropriate in the circumstances, as opposed to a driver’s compliance with signals.

Indeed, that is one reason why traffic engineers keep raising speed limits; they know that since people do not comply, the only way to keep people within any reasonable approximation of speed limits is to constantly raise the limits. This is not true everywhere. In places like Finland traffic speeds have been reduced to 50 km on connectors and 30 km on residential streets and speed limits are ENFORCED. More and more places are using “day fines” (fines based on the individual’s daily earnings so that no one is left with the feeling that speeding is just a matter of paying a nominal fine). Finland has all but eliminated death or injury from motor vehicles.

The NTSB indicated that the most effective ways to address injuries to bikers and pedestrians is infrastructure properly designed (something we, in Alaska, have NEVER DONE), and ASE (automated speed enforcement), which Anchorage has historically viewed as an unwarranted intrusion on the freedom to kill and maim.

In Anchorage, the common wisdom is that traffic signalling is advisory only. As a result, there being virtually no enforcement, the only way to implement appropriate traffic design to protect bicyclists on roads where traffic exceeds 30 mph is physical barriers. Unfortunately, though local designers give lip service to “Vision Zero” and the like, we have no cycletrack.  In fact even the sidewalks are designated as snow storage. And while northern cities in North American and Europe seem to manage snow and cycletracks just fine, you will not see any cycletracks in Anchorage because designers will make sure they are never built. Apparently, they argue, we are not quite ready to stop killing and maiming pedestrians and bicyclists.

I posed that question to the Assembly here once. How many bodies do we need to count until we do something about this. They would not give me a number, and mumbled something about how unfortunate it is. They then essentially patted us on the head and told us to go back and play in the traffic.

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