vrkelley

Big mystery, How do drivers judge a bike's velocity? If drivers have bad depth perception, how can I proactively convey my speed to avoid hits?

z90

When you are going fast downhill, keep your feet moving, don't coast.

jputnam

A few things that can help: reflective rim markings, a larger visual signature, and steady (not blinking) lights.

It looks dorky, but put a few pieces of reflective tape on your rims, together in one part of the rim, say two or three spoke gaps. This makes a moving patch of light that moves faster as your wheels rotate faster. If you cover too large an area, the light will become visually constant at higher speeds.

If you ride with a pannier, have good reflective material on the pannier. If you're picking out a rain jacket, look for one that has reflective material on the sleeves as well as the back. Look for shoes that have reflective material; tights that aren't solid black, etc. Human vision is better at judging the distance and speed of larger objects than small.

Blinking lights are great for grabbing attention, so by all means use them if they're legal in your area. But don't rely on blinking lights alone for night-time conspicuity. It's very difficult for many people to judge the speed and direction of blinking lights. Complement them with a steady light or large areas of reflective material to provide a continuous light for the eye to track once the blinking light draws attention.



The blinking LEDs may be what first draws attention to my commuter rig, but once you've noticed me, your eyes will have plenty of steady light to track me with.

Finally, don't cower in the gutter. Get out in the lane where bikes belong so drivers notice you sooner and recognize you as a vehicle before they're right on top of you.

Bekologist

Running high powered lights, on flash during the day (or night IMO), will assist motorists in identifying you as a moving object carrying momentum. John Duggan, local NW bike lawyer, has written an article endorsing the use of daytime running lights as an aid in cyclist safety. reports from numerous hard core NW commuters support this with their personal observations about the value of high powered LED arrays for roadway safety.

i am confident -as are many others- that these daytime visible, high powered LEDs assist cyclists in broadcasting their lane presence and momentum/speed to other road users.

njkayaker

Lights allow bicycles to be identified as something as being present/moving much farther off. Since motorists tend not to expect bicyclists, it seems lights would have to improve things.

crackerdog

At night, do not use a fast blinking light- that just completely throws off my depth perception. The little bright lights on a bike at night do nothing to give distance or speed. Look at a car, is it the headlights you look at? No, it is the light hitting the road and surrounding area that gives the clue to where the car is and how fast it is going. That is why something like Down-Low-Glow is so great (but their battery is crap and I wouldn't buy one until they fix it). Take a good bright flashlight and aim it down right in front of the bike or even aiming at the front wheel. When you are out at night, watch other bicycles and pretend you are a driver that doesn't know about bikes at night. Do you immediately know what that little light is?
During the day a bright strobe is great and especially at dusk or dawn when there is lots of light in the sky but the ground is getting dark- the eye is adapting to the sky but the danger is on the ground. Ask a motorcyclist what works for them. They get ignored by cars all the time and they run with their headlights on during the day and are much larger than a bicycle.

Roughstuff

As any train engineer can tell you, depth perception when you are looking 'straight at' an object coming toward you (a bicycle, or even a locomotive) is among our worst judgements. I agree visual cues like pedaling and increased reflectivity help, but in general I assume the other driver will misjudge my distance and speed. I look at my responsibility to ride defensively to be as great as their responsibility to properly judge my location....if not even more so!

roughstuff

Keith99

Are you a decent rider and ride 18mph or higher?

If so the only thing I can think of is ride only where the racers train!

I'm serious, as others have pointed out judging speed straight on is difficult and if you come in with the idea that bikes go the speed of a clunker with someone who is just cruising then by the time it registers on a driver that you are not fitting his preconception it is often too late.

Just a thought. I'm far from a racer, buit I can not recall a single case where a driver has misjudged my speed thinking I was going slower than I was. On the other side I get passed fairly often by cars who want to make a right and cut in front of me, and every time I can remember, save one, it actually would have been a safe pass, the car could have passed and reestablished itself in the lane before the turn, if I was going as slow as most clunkers go.

The one thing I do a bit is to slightly slow in situations where I think problems might occur. I can not change the general preconception, but I can reduce the difference between my speed and what they expect. I don't brake, I just sit up a bit and soft pedal (and sometimes make sure I am ready to brake).

Pat

True depth perception comes from the fact that humans have 2 forward oriented eyes with a gap between them. The difference in the angle to the object from the 2 eyes allows the brain to estimate the distance to the object. Human depth perception only works out to a maximum of 10 meters. So it really does not come into play in estimating the speeds of approaching cyclists.

The way people would usually estimate the speed of approaching cyclists is the movement of the cyclist compared to their surroundings and/or a change in the apparent size of the cyclist. Blinking lights would sort of disrupt that because they would interrupt tracking. However, blinking lights are far more noticeable than constant lights. Human vision tends to notice changes in the surroundings much more readily than constant things. So a blinking light would make the cyclist far more noticeable even if it made guaging their exact location and speed more difficult.

chandltp

I would assume two headlights would be helpful, on constantly.

Roughstuff

Wow. Then again our the distance between our eyes is a pretty small component of an isosceles triangle at that depth.

I remember when I ran X-country I was always encouraged to make sure you 'stayed in contact' with the person in front of you as much as possible. Once out of contact, it was hard to motivate yourself to pursue a runner whose distance was hard to judge. Same effect, I imagine.

roughstuff

jputnam

Motorcyclists often use lights that rapidly cycle a few percent dimmer and brighter -- the same effect as using both steady and blinking lights. The variation draws attention, the steady light allows better judgment of position and speed.

Somewhat amusingly, that started out as a defect of motorcycle lighting systems back when most bikes had undersized batteries and magnetos -- the charging system varied the voltage that widely with every rotation of the motor. When motorcycles got modern electrical systems and the charging voltage smoothed out, people started adding circuitry to recreate the modulated headlight intensity.

jputnam

Some studies suggest a possible problem with that setup -- will a driver mistake your narrowly-spaced bicycle headlights for widely-spaced car headlights far away?

chandltp

I forgot to mention that they should be mounted on 8 foot wide handlebars

Seriously, I think the mistake could be made but it does provide additional visual information. I think in reality most cars assume bikes are going slow and drive as such.

Pat

Well that is part of the problem isn't it. I think most drivers tend to see things as "pedestrians" who are for most considerations motionless and "motorized vehicles" going at say 40+ mph. I have seen many drivers look at me and sit. Then they realize that I am not going 45 mph and they start to pull out but at 20 mph or so, I am going fast enough to frequently be a "problem". I think many drivers have a problem with the fact that cyclists are not going the speed of traffic but they are not virtually motionless pedestrians. At 20 mph, a cyclist is going at around 50% of the speed of traffic. It is a difficult problem for drivers who have not seen many cyclists.

JoeyBike

If you are riding in busy urban/city/grid conditions, stay away from the gutter as much as possible to avoid crossing traffic running stop signs on you. The closer you get to the curb, the less time they have to judge your speed and the less time you have to react if they run the stop sign anyway.

As a rule, on roads up to 35 mph with or without on-street parking, I ride between 18 and 23 mph (easier for them to detect speed if you are holding near normal traffic speed) and put my wheels in the LEFT car tracks of the right lane - just like a motorcycle would do - assuming nothing is coming behind me. I use a rear-view mirror and glance in it every 10 - 15 seconds. If the cars coming up behind me have room to move over one lane, and are doing so, I stay put (there are no minimum speeds posted) and they just go around me. If there is a conflict behind me, and it is reasonable for me to move over and stay out of the door zone, I will move right JUST OUTSIDE the door zone and actually USE the cars passing me as a rolling blockade. I KNOW the crossing cars will see and respect those vehicles.

If you are hugging the right curb or the parked cars: A. You will be physically harder to see for the crossing traffic drivers, B. It will be impossible to judge your speed from that angle, C. Even if the car sees you, recognizes that you are on a bike and moving fast, they will STILL BELIEVE they can race across your line to cross in front of you (they are, after all, more important road users). If you are far LEFT, they will realize they can not make it all the way across without killing you and virtually 100% of the time (in my experience) will just allow you to go past. I give them a nod or a wave of thanks if I can.

On wider, faster roads or if you can not cycle near 20 mph, you better be ready to turn or hit the brakes. If you are riding under 18 mph, you are also increasing the chance of right hooks. So they get you both ways - crossing and overtaking. Learn to ride faster, find another route, get a rear-view mirror and learn to use it just like all of the motorized vehicles are required to do for very good reason. And for all else, be prepared to change course or stop - pay attention. No wool gathering. Remember George Carlin's line: Imagine how stupid the average person is, then realize that half of them are dumber than that.

genec

I have to say that this is a pretty good and fair assessment of the realities of urban/city cycling... you know the kind of place where messenger style riding is king. (or Joey style riding)

This is also somewhat counter to the rantings and ravings of strict vehicular cycling -- the fosterites, who time and time again tell us that speed doesn't matter. The reality is speed DOES matter... it matters enough so that traffic flow is the key factor in the how and why of traffic engineering in our cities. (safety is secondary to flow) This is also why your grandma and my grandma will never be utility cyclists in America (unless they use back streets to get from the residence to the market... if said alternatives exist).

Interestingly Forester tells us that on one hand speed doesn't matter, but on the other, speed differential can make "negotiation" between cyclist and motorist nearly impossible, and that the "dedicated cyclist" prefers to go fast.

I have to fully agree with your comments about cycling speed on roads up to 35MPH, and contend that this is why higher speed roads, such as those seen in places in the west like southern California, with typical arterial roads of 50MPH and higher, are not conducive to cycling. Oh it can be done, but as a "road sneak" or with the end result that you either fight FRAP laws or impeding laws (like Chipseal and others) or you prove you were actually riding at the posted speed (a'la the Brentwood doctor case... where cyclist SPEED was a key factor in the case against the motorist)

Theory verses reality... reality wins yet again. Thanks Joey. Keep spinning.

vrkelley

Still recovering from the car/bike accident. Thanks for the advice. Any other takers (advice from other riders)?

5b00

This is incorrect. Motion parallax is another mechanism, and quite possibly plays a larger role when things are moving. This is one reason why people with vision in only one eye can still perceive depth. The use of a constant light probably aids people in judging distance through motion parallax, as it provides an easy-to-see reference of the cyclist's position over time. Then again, other things like the cyclist's head could also fill that role.

Giro

At night, using two lights spaced reasonably wide apart provides better depth/distance perception than a single point source light. This is because as you get closer, the angle an observer such as a motorist sees between two points increases. Up front, a handlebar light and a helmet light would be one method to achieve this. In back, a helmet mounted tail light and a bike mounted tail light.

Other depth perception aides at night could be a standard ANSI-class reflective vest (since the sizes of these do not vary much), reflective tires, and some Dept of Transp alternating red/white reflective tape.

During the day, probably getting noticed more than momentarily should help but other than bright clothing and a fairly bright flashing front and rear light, I can not think of much. Most importantly, ride defensively as discussed in numerous threads on these forums.