At what point does a road become too busy/fast to ride on safely?

@Rider_X has already given a good answer.

I would add an important consideration: local driver and cyclist cultures.

As you and the map you cite indicate, there is little pro-cycling culture in this area. If you don't see other cyclists riding there, then don't do it yourself.

The road you picture is quite poorly designed, even for motor traffic. That center turn lane is recognized world-wide as a recipe for head-on crashes, and the lack of any run-off at the roadside means that breakdowns or vehicle stoppages will cause potentially fatal obstructions.

If it's not safe for motor vehicles then it's not safe for you.

As a counter example, where I live on the edge of a city of 4 million people, my ride yesterday evening started on a two lane road (one each way) with an 80 kph (50 mph) limit and bike lanes, and after 10 km (6 mi) switched to 100 kph (60 mph) and no bike lane. But crucially, there is a verge that can be used as an escape. Of course, I had to take care, and especially listen for vehicles approaching from behind. But every single vehicle that passed me did so with at least two meters to spare. At one point I heard what turned out to be a 24-wheeler approaching, and moved onto the verge (which was deep loose gravel). The truck driver honked to thank me after he passed. My point here is that in this area the culture is for cyclists to be reasonably well respected road users.

Cyclists are vulnerable when mixing with other traffic. If you have to ask is it safe then the answer for you is no.

Like the other answers have said safe bicycle commuting is subjective and undefinable to a large degree. When I lived in Colorado a child was killed in a crosswalk on a bike while riding with their dad. In contrast, I've ridden my bike on pretty unsafe roads¹, and so far I'm unscathed. I'm sure a large part of this good luck on my part and tragically bad luck by that child. I bring this up to highlight that accidents might or might not happen regardless of where you ride. This doesn't mean you should flirt with danger but you should accept that commuting on a bike carries risk.

Based on the picture you showed I personally wouldn't have a problem riding on it, but I would only ride on it if there wasn't a better option. Keep in mind my judgment can be a bit suspect¹, and both other answers have essentially recommended against it. If you do ride on that road be mentally prepared to have cars whiz by you pretty fast. That can be an unnerving feeling at first but it isn't bad once you get accustomed to it. It is important to keep riding your line in that situation though. If you get nervous and start swerving around when the cars whiz by that is increasing your danger. Also it would be better if you would commute during off-peak hours.

Like another answer mentioned I think it is important to consider the bike/car culture in your area. I have a friend who has lived all over the world. He once told me that Washington DC feels much more dangerous to bike commute to him than Kenya did. I currently live in Idaho, and the drivers are almost excessively accommodating when I'm towing my kid trailer. You should adjust your boldness based on how the cars react to your presence on the road. In my city people would give me my own lane on that road even if I didn't demand it, and in Washington DC I'd probably be driven off the road if my friend is to be believed.

In summary, I personally don't feel like that road looks exceptionally dangerous, so I would ride on it unless the traffic is really unfriendly. That being said the only one that really needs to be comfortable riding on it is you. To repeat another answer, if you don't feel comfortable riding on that street then you shouldn't be riding on it. If you do ride on it then accept that cars will rush past you at close range.

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1: I rode my bike out of Amsterdam on the autobahn, because that was the directions people kept giving me even though they saw me on my bike. After about 10 miles and strange looks from passing motorists the police pulled me over. Fortunately for me they just yelled at me and escorted me to the next off ramp, but I'm certain I could have been at least given a heavy fine. I also weaved through traffic in a huge roundabout in Barcelona that was filled with 20 - 30 mph bumper to bumper traffic merging from what felt like all directions. I was strong at the time so I wasn't too far below the flow of traffic, but my cousin said he was sure I was going to be hit. Both were stupid moves and I was lucky not to be hurt.

I need to ride on a road exactly like that. Looks exactly like Florida. I have an 18 inch side which appears a bit bigger than yours which is the bike lane. Then there is the berm which is what they call the grass. I need to get over 30 miles on this road to get to a destination I am interested in. So, it is too far for me to hike in a day. I have driven on this road and vehicles achieve speeds of 65 MPH+ regularly. There are no streetlamps so it is very dark at night. I have seen cyclists lazily peddling in that small lane without much trouble.

When I need to I plan to ride on the berm (grass) as much of the way as I can. I have tried it with my mountain bike and I am stable on it, at least for short distances. I notice you have a bit of sidewalk on the left so I would use that whenever I could. I would not trust to ride in the small bike lane provided. One mistake from that idiot behind you and you will be a red splotch.

I would advise that you map out this road very carefully by driving over it or even hiking it. I have seen roads in the southwest narrow until there was no bike lane or grass to use and that I consider a death trap. Fortunately, we have plenty of berm here in Florida because some people here hike and trails sometimes follow roads.

One more point I might add, drivers here despite that poor reputation know about and respect bikes. This has been improving because of the bad record Florida has maintained for years and police crackdowns. The Southwest was much worse, particularly Vegas where drivers would come up on the curb to hit a biker(happened to me twice).

Be safe and good luck!

Statistically speaking you are much more likely to die from skydiving. Or, since someone is going to say, "but we don't parachute", is that more likely to being hit by a skydiver belonging to an unopened parachute? All seriousness aside, there are several things that a person is more likely to die from than cycling no matter the road. But then the question is which roads are more dangerous than others and maybe should be avoided. I like Rider_X's statement that the decision is a personal assessment issue because it really is just that.

Generally

I can only speak for myself and like me, some cyclist are comfortable on any road, at almost any conditions. If your alert and always aware of what is going on behind you then the risk is minimized. But then I have been riding on roads of every type for for 38 years. We can call this type 'A'. However, other cyclist, especially ones new to cycling, are understandably cautious and nervous around motor vehicle traffic. Especially thicker traffic on busier roads at higher speeds. We can call this type 'B'

Comfort does come with experience, confidence and mileage. But that doesn't mean that EVERY cyclist should bide on EVERY road at EVERY time. Since type 'A' feels very comfortable , he is less likely to make a dangerous mistake or do something unpredictable to confuse motorists and he feels perfectly calm and at ease. He is accustomed from almost 300,000 miles of bicycling to know where passing cars are from the sound of their tires hitting the pavement. He doesn't have to even look behind him or have a rear view mirror (Not advocating not having a rear view mirror) to know if motor vehicles are getting too close. Whereas, rider type 'B' does not have the same experience and confidence. Bottom line to this story is that even though I am comfortable on just about any road, there are still some roads and condition I try to avoid at certain times.

My experience from tens of thousands of miles

I have always found that the majority of motorist are alert enough to see and steer clear of slower moving vehicles. I used to say it was 97% and the other 3 percent had to not be paying attention just as they pass you and they may still miss you. But with today's handheld devices, motorist are more distracted than ever. Its not unheard of to see a motorist driving down a road with a device in from of their face while moving which is illegal in every state in the USA and probably every country I would imagine. All that being said, the motorist that i have the most problem with by far are the one that either don't think bicycles are allowed on the roads or should be on the roads or just upset that they have to deal with a bicyclist and try to come as close as they can to you on purpose as if to try and make a statement. Those are the ones you have to look out for the most. Even though they are paying attention, since they are trying to get close, they could get so close as to cause a critical mistake.

Your particular Road

That particular road does not look that long in the picture if your in the middle taking the photo. It does not have any shoulder to dive off on though and if there was another road nearby that was better, I would take it. Even if it takes you a half or even 1 mile out of the way. The average traveling time by bicycle to cover a mile id roughly 3 minutes give or take 30 second depending on if your faster or slower than average.

My criteria is to assess as many things as possible aside from my own comfort level as:

1. Thickness of traffic at the time of day I plan to use it.

2. Will I be commuting while near dawn or dusk and how lighted is the road.

3. Are there blind hills and curves

4. Speed of traffic

5. Type of driver predominately traveling this particular road

6. Road condition including pavement style, evenness, age (cracks and pothole), existence of a shoulder, layout, design,

7. Type of vehicles using it. How many larger vehicles like trucks aas compared to the number of cars.

Not an all inclusive list by any means.

One last notable thing - LIGHTS

We have a retired guy in our club that rides with his lady friend on a tandem and they have about eight lights on their bike that they run at all times. With today's LED lighting you can have lights that are so bright they will make you stick out in the day time like a riding lawnmower going 70 mph on an interstate. My friends have been told by several motorist that they were almost blinded by their light in the day time. So you can always make yourself much more visible.

First you need to check your local laws. Depending on where you are, there may be rules about "how" you ride on that road. For example in FL you are treated the same as a car. If you hit a person on the sidewalk, and they decide to press the matter, your classified, just about the same as a if you drove your Honda onto the sidewalk. Local bike clubs and what not can help with understanding local laws.

Now, just because you have the same responsibilities as a car, and the same liabilities as a car, doesn't mean your going to fair as well as the car that hits you. So keep that in mind.

Follow the simple rules (these are for my area, you may need to look up your area)

• Ride on the correct side.
• Take up an ENTIRE LANE (go ahead, piss 'em off ride in the middle of the damn lane, you allowed and supposed to. The official rule is that you have to ride far enough into the lane, that for cars to go around you they have to change lanes.)
• Use the correct lighting. We have rules here about how bright and where lights must be.
• Keep up with traffic. Now this is the important one for you. Again, here, you are legally required not to pose a impediment to traffic, while also driving in the middle of the lane. So if you can get your bike up to 40 - 55 MPH go for it. Otherwise your breaking the law (and you will get ticketed).

Me personally, not going in that road. It seems unsafe, and I don't see how I could meet the local laws (there's no way I'm getting my bike anywhere near 40MPH). I would look for alternative roads and paths. Google maps does a pretty decent job around here of highlighting "bike friendly" roads. But then again the city I live in puts bike lanes on main roads if the speed limit is above 35.

I have ridden roads like this, but only in large herds groups (30+ people) where we were able to draft and maintain 30+MPH in 55+MPH traffic. Also, there was a generally agreed upon "they'll have to get us all" mentality that if anyone was hit, the rest would ensure aggressive legal persecution prosecution would ensue. This ride was in the Gainsville, FL area back in the late 80's.

It was very nerve-wracking for the first few miles, then it got more comfortable. We were always very conscious of the traffic, and had constant communication through the group of traffic approaching from behind. As I recall, we also had a bit more shoulder.

Riding solo, I wouldn't touch it with a 10-foot pole!

I appreciate you're asking for advice, and unfortunately I can't help. As others have answered safety is a very subjective experience. But I would like to explain what I think is a definitive answer from the perspective of the engineering and design, coming from the Netherlands, which has the safest and highest levels of cycling in the world.

The Dutch have the principle of Sustainable Safety, the objective of which aims to "lead to a sustainably safe traffic system in which: (serious) crashes are prevented, and, where this is not possible, severe injury is almost totally prevented" (see this fact sheet or this blog summary for more info).

The CROW Design manual for bicycle traffic is the infrastructure design guide used in the Netherlands, and specifies the default minimum of safe design for cycling. In it there is a simple table that specifies whether cycle traffic can mix with motor traffic given the maximum volume of motor traffic (measured in PCU, Passenger Car Unit), the maximum speed limit of the road, and the number of motor traffic lanes.

Using this table we can see that the point at which a road becomes too dangerous to allow mixing of cycling and motor traffic is when a road is

• a rural access roads with…
  • a speed limit of 40mph or more
  • AND more than 3000 PCU per day
• an urban distributor of…
  • more than two lanes in one direction
  • AND a speed limit of more than 30mph
• a rural distributor
• any through road

The types of road are planned and defined as follows (source: pedestrianiselondon.tumblr.com)

1. Access roads (aka estate access roads) - streets that provide access to destinations, non-through routes to motor traffic.
2. Distributor roads (aka district access roads) - streets that provide access to estate access roads, non-direct through routes.
3. Through roads - roads that link distributor roads together and act as main routes for motor traffic.

From your image it's clear that the road is not within the Dutch safe limits even if the volumes of motor traffic were low. Were the CROW manual followed, an cycle track (which would be at least 2 meters of width per direction of bicycle traffic, physical separation from the road of something like at least 2 meters, and noise reduction), or an access road (that would typically be access only to motor traffic and thus be nearly free of cars) should be made.

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Please note that the Dutch standards and implementations for both on-road and off-road cycle tracks, while not universally perfect, are much higher than in other countries. The general design of roads is more thorough too, which typically ensures the volumes and speeds of motor traffic remain as planned.

My completely subjective non-scientific UK guidelines

Sorted by car speed limits. I'm riding at 25/30 km/h (15/19 MPH) on a cheap touring bike. Please don't sue me if you crash.

• 30 MPH: always fine. These are smaller roads linking smaller towns or larger roads inside built-up areas (30 MPH is the national limit in built-up areas)

• 40 MPH: avoid if possible, but usually it's OK.

  At this speed, things are highly dependent on how busy the road is. If it is a country road linking two small towns, it will be completely fine.

  But if it is a busy road between larger towns with cars constantly moving, then it quickly becomes unacceptable.

  Other factors to consider:

  • if going downhill, things are less bad, as you reach speeds more comparable to that of cars. And if going uphill, things are worse, conversely.
  • if you are in a group, you become more visible than alone, and are therefore safer
  • do you have a day-visible flashing backlight?
  • if there is some kind of pedestrian path, even if it is not ridable, things are better as you know you can use that as a backup in case of a bicycle/body problem

• 50 MPH: almost always avoid, unless you know the area very well and are sure that it is a low traffic region, and that it is not going to be riding there for very long.

  Such acceptable locations do exist, but they are the minority of cases, and you really have to be sure about it. Sometimes you left wondering: how can this possibly be 50 mph, it is such a quiet location/small road! Who designed this?!

  If you have to use one of those for small transitions, you will generally have to push your bike on the grass by the road, and any non-pedestrian bridge crossing (which won't exist in the middle of nowhere) will be an adrenaline rush.

  When you see that 50 MPH sign on a place you've never been before, be smart, stop and look for a secondary path on your map. Otherwise there's a good chance you will be staring at the continuous white line on the road side trying to stay as close to it as possible for a few tense kilometers as cars swoop down a few centimeters away from you!

• 60 MPH: always avoid.

  It does appear to be legal to ride in certain of those roads: it is only illegal to ride on motorways (M*) roads, but there are A roads which reach up to 70 mph (if they are dual carriageways), see also:

  • https://en.wikipedia.org/wiki/Road_speed_limits_in_the_United_Kingdom#National_speed_limits
  • https://en.wikipedia.org/wiki/Great_Britain_road_numbering_scheme#A_roads

  However, even if it is legal, you would have to be insane to to that!

  Luckily however, roads are setup in a way that I have never accidentally gone into a 60mph road by mistake, unlike 50mph which I've done a few times.