# Passing distance vs distance from curb

Study shows gutter bunnies get squeezed; for maximum passing distance, ride three to four feet from the curb.

The Florida Department of Transportation published a study [PDF] in which highway researchers measured how much room car and truck drives gave cyclists on Florida roads.

Florida’s study seems to replicate the “Mary Poppins Effect” first observed by Dr Ian Walker at the University of Bath, England in 2005. Walker found motorists gave him more passing room when he wore a wig (to appear like a woman) and when he was casually dressed (vs in athletic apparel). The FDOT study found similar results.

A little more interesting to me, and probably more relevant from a safety standpoint: FDOT researchers found car and truck drivers gave more passing room for cyclists who ride further out from the curb, up to a certain point.

Intuitively, one would expect that the closer you ride to the curb, the more lateral separation you have. On the contrary, the results presented in Table 4 and Figure 6 show that riding closer to the curb results in a smaller separation. Field observations revealed that when bicyclists ride closer to the curb, some motor vehicles, especially compact cars attempt to fit in the lane without laterally shifting to the adjacent lane, hence causing lesser distance.

On the other hand, the results show that riding too far from the curb also results in a shorter distance. It seems that there is a spot between 3 and 4 ft from the curb that results in the greatest lateral separation between motor vehicles and bicyclists. It should be noted however, that higher standard deviations were observed. This was mainly caused by the fact that some drivers choose to stay within the outside through lane while others laterally shift to the inside lane.

Other findings:

• Passenger cars give less passing distance (mean 5.19 feet) than SUVs and pickup trucks (5.30 and 5.25 feet, respectively)
• Large trucks give the most passing room at 6.27 feet.
• Box trucks give the least passing distance at 4.48 feet. Florida bus drivers give the second least at 4.79 feet.
• Drivers slowed slightly when passing cyclists, dropping their speed by an average 1.4 MPH, but sped up and drove faster than their previous speed after passing, moving 2.7 MPH faster than previously on average. This “drive faster” behavior is common for traffic calming measures such as speed bumps and stop signs; as far as I know this is the first time this has been observed when passing cyclists.

From Bike San Diego, which has more discussion on the gender and apparel aspects of this study. Via Streetsblog Network. Study authors were Thobias Sando, h.D., P.E., PTOE
and R. Moses at the University of North Florida.

Photo by Bruce Dean.

1. Anonymous says:

Not having had time to read the 97 page report yet, how much greater were the standard deviations? And was the passing distribution normal or skewed somehow?

2. Standard deviation ranged from 1.2 for the gutter bunnies (min 2.1 feet to max of 11.6 feet passing distance) to 1.6 for those riding 5 to 6 feet from the curb (min 2.8 feet / max 9.7 feet).

I thought I saw in the study narrative that distribution was “near gaussian” (i.e. almost a bell curve) but I can’t find it at the moment.

Specific discussion on passing distance relative to bike distance from curb begins on page 12.

3. Anonymous says:

It would have been helpful if they’d also presented it as “feet” from left lane boundary. Four feet from the curb is a lot different in a 10 foot lane than a 20 foot lane.

4. Yes true. The published paper has tables showing lane width and passing distance but I haven’t teased through all of that (yet).

5. ianbrettcooper says:

The study bears out my experience. Since I received training as a cycling instructor, I’ve always ridden about 5ft from the curb (6ft or more away from parked cars to prevent “doorings”), which is what the League of American Bicyclists recommends to prevent unsafe passes when there’s oncoming traffic (and it gives me plenty of room to move to the right if someone is going to overtake too close when there’s no traffic). I find that riding any closer to the curb invites people to try to squeeze by within the lane.

The problem I’ve found is that when I ride that far out when there’s no oncoming traffic, motorists sometimes decide to “teach me a lesson” by passing too close on purpose. But at least I can move to the right – if I were to hug the curb as many cyclists do, I can’t move to the right if someone tries to overtake too close.

I suspect a lot of cycling injuries and deaths come, ironically, from riding too close to the curb. Motorists either don’t see the cyclist because he’s lost in the curb clutter, or they try to squeeze by and if they misjudge, with the result being a dead or badly injured cyclist.

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