Rear lights, and what a radar tail light actually tells you

Front lights get the attention because they are the ones you look at. The rear light is the one that matters more, because the collision type it protects against is the one you cannot see coming.

What makes a rear light effective

• Visibility angle. A light visible only from directly behind is no use at a junction. Look for a stated angle of 180 degrees or more, or a lens that wraps.
• Height. A seatpost light sits at about driver eye height. A light on a rear rack or a bag sits lower and is more easily hidden by your own body.
• Mode. A slow pulse is more noticeable than a steady light and less disorienting than a rapid strobe. Rapid strobes make it hard to judge distance and are genuinely disliked by other riders.
• Daytime output. A dedicated daylight mode — very bright, short flash — is measurably effective on bright days, which is when a lot of collisions happen.

Two rear lights, not one

The argument is simple: one will be flat or will have fallen off, and you will not know. Two, in different places — seatpost and bag, or seatpost and helmet — costs little and removes the single point of failure.

Run one steady and one flashing. The steady one lets a driver judge your speed; the flashing one gets attention in the first place.

Radar tail lights

A radar tail light watches the road behind and tells you, through a head unit or your phone, that a vehicle is approaching and roughly how fast.

What it genuinely does: gives you several seconds of warning on a quiet road, so you can move, take the lane, or simply be prepared. On rural roads and long straight stretches this is a real change in how relaxed the riding is.

What it does not do: make you visible. It is a tail light with a sensor, and the light itself must still be bright and wide-angled. It also does not brake for you, and it will not help in dense traffic where it reports a continuous stream of vehicles and becomes noise.

Who it suits: riders on quieter roads, riders who are anxious about traffic behind them, and anyone who rides alone. Less useful for city commuting.

The honest cost question

A radar unit costs several times a good conventional tail light. If the budget is fixed, two good conventional lights plus reflective clothing does more for visibility than one radar unit does. If the budget allows both, the radar adds information you cannot otherwise have.

We will say that at the counter rather than sell up.

Reflectors still matter

A rear reflector works when your battery is flat, which is exactly the moment you need it. It costs almost nothing and weighs nothing. Keep it, even with good lights.

The same applies to reflective material on moving parts — ankles and pedals. Movement is what the eye picks up, and a reflective ankle band is more visible at distance than a static reflector on the frame.

Mounting where it can be seen

Check that your saddlebag, rack or jacket does not cover the light. Sit on the bike, have someone stand ten feet behind at eye level, and look. Half the rear lights we see obscured are obscured by the rider’s own bag.

Winter and salt

Road salt on the North Shore is hard on contacts and clips. Rinse lights occasionally, dry the charging port before plugging in, and check the mount for corrosion. A light that falls off in February is usually a mount that rusted in January.

Weekday counter

We are open Monday to Friday afternoons. Bring the bike if you can — mounting position is easier to solve with the bike in front of us than by describing it.

Brake-sensing tail lights

Some rear lights include an accelerometer and brighten when they detect deceleration, in the way a car brake light does. It works reasonably well and it is not a substitute for a wide, bright steady light.

The one thing to check is false triggering: a light that flares every time you go over a bump is distracting for the rider behind you and quickly becomes noise.

Mounting on a seatpost with a saddlebag

The most common cause of a rear light doing nothing is a saddlebag hanging directly over it. Fit the light, sit on the bike with the bag loaded, and have somebody look from ten feet behind at eye height.

If the bag covers it, move the light to the seat stay, the rack or the bag itself. Many bags now include a dedicated light loop for exactly this reason.

Two lights, different modes

The arrangement that works: one steady, one slow pulse, mounted at different heights. Steady lets a driver judge closing speed; the pulse attracts attention. Two rapid strobes together are worse than either alone — they make distance judgement genuinely difficult.

Group riding etiquette

If you ride with others, switch the rear light to steady when you are not at the back. A strobe directly in front of another rider’s eyes for an hour is unpleasant and, on a dark road, actively unsafe.

The same applies to a bright front light on a shared path: tilt it down or drop to a lower mode when someone is approaching.