Why this is harder than it looks
Every time you cross in front of a car, pass a pedestrian or decide whether you can stop in time, you are making a judgement about speed and distance. Most of the time you do it without thinking. It feels like seeing, but it is really estimating, and estimates can be wrong, especially for fast-moving objects, in poor light or when you are tired.
The sensible response is not to try to become perfectly accurate, which is unrealistic, but to understand where the mistakes come from and to build in margin so that an error does not become a collision.
How we estimate
Roughly speaking, the brain judges distance from how large an object looks, how it sits against the background, and how quickly it seems to grow. Speed is judged from how fast that apparent size or position changes. Some points follow from this:
- A distant vehicle that is approaching directly changes its apparent size slowly at first, then quickly at the end. It can look further away and slower than it really is, and then arrive sooner than expected.
- Small or narrow objects, such as a motorcycle or another bike, can seem further away than larger ones, because the cues we use are weaker.
- Vehicles with a slow, quiet approach, or ones coming head-on with little side movement, give fewer clues.
- Darkness, glare, rain and low sun reduce the information available.
None of this makes you a poor judge. It simply means everyone’s estimates have limits, and the limits are larger in difficult conditions.
Judging gaps
When you want to turn across traffic or cross a road, you are really asking whether a gap is big enough. A few general habits help.
- Choose gaps that feel generous. If you have to ask whether you will make it, the answer is often to wait.
- Remember your own acceleration. A rider starting from a standstill, in a hard gear or on a slope, needs more time to clear the road than the same rider already moving. Having an easy gear selected before you set off, as described in starting gear, helps you pull away promptly.
- Allow for your bike and load. A heavier bike or a bike with a child seat or trailer can take longer to get going and longer to stop.
- Watch for acceleration by others. A vehicle that looks slow may speed up, and one that looks fast may slow, so look again just before committing.
If a driver is waiting and seems to be giving way, treat it as a possibility, not a promise. They may have intentions you cannot see.
Closing speeds
When you and another road user approach each other, the speed at which the gap shrinks is the combination of both speeds. Two vehicles each moving at a modest pace can close a gap very quickly. A simple example: if you are riding at an easy pace toward a car that is approaching at a higher speed, the gap shrinks at the sum of the two speeds, so a space that looks comfortable can disappear in a few seconds. The practical point is that head-on and crossing situations are shorter in time than they feel, so it is generally better to decide early.
Wind can also change how fast you are going without you noticing. In a strong tailwind you may be moving faster than it feels, and a headwind has the opposite effect, as noted in windy day riding.
Stopping room
Stopping takes time and distance: first you notice the problem and decide to act, then the brakes work. The total depends on your speed, your reaction, the bike, your brakes, your tyres and the surface. Without quoting figures that vary so much from rider to rider, these points hold in general:
- Going faster increases stopping distance by more than it feels like it should.
- Wet surfaces generally lengthen it noticeably, which is why wet road traction matters when judging how much room you need.
- Loose or slippery surfaces and a loaded bike also need more space.
- Braking technique matters. Smooth, firm use of both brakes, covered in front and rear brake use, generally stops a bike more effectively than grabbing one lever.
The science of what changes stopping distance is explored further in braking distance factors. In practice, the question to ask is: “If the thing ahead did something unexpected right now, could I stop or steer around it?” If not, ease off.
Why extra margin helps
Because estimates are imperfect, margin is the thing that absorbs error. A bigger gap, a lower speed and an earlier decision all buy you time. Margin is cheap when you give it and expensive when you do not have it. Some riders find it helpful to think in terms of a buffer: a space around themselves that they try to keep clear where possible, and that they enlarge when conditions are poor or the situation is unclear.
Practising the skill safely
You can train your judgement without taking risks.
- In a quiet, traffic-free area, place a marker and practise stopping at it from a gentle speed. Note how far past it you go, then adjust.
- Try the same at slightly different speeds, always staying within a speed that feels comfortable and controlled, and always wearing a helmet.
- While walking or as a passenger, practise guessing how far away parked objects are, then check by pacing. Seeing how your estimates compare helps calibrate your eye.
- Watch traffic from a safe spot and try to guess when a vehicle will reach you, then check.
Over time you learn your own tendencies, and whether you usually over- or under-estimate. Where in doubt, slow down and wait; it is rarely a mistake.