The same barrier can have more than one of the characteristics listed below, and they may be different for different types of vehicles.
- Containment: The barrier prevents a vehicle from, for example, going over a cliff, or straying into oncoming traffic. Examples: curbs, guardrails, Jersey barriers.
- Deflection: The barrier guides a vehicle that has gone off course back into the intended direction of travel. Examples: Jersey barriers, guardrails.
- Threat: The barrier is hazardous in itself, so drivers shy away from it. Example: boulders, rigid bollards.
- Sham: The barrier appears to pose a threat of damage to a vehicle but in fact is designed to minimize or avoid damage. Example: flex posts.
- Stop: The barrier is intended to stop a vehicle approaching it.
- Energy-absorbing: The barrier is designed to lengthen the time and so decrease the severity of an impact — same idea as with air bags or helmets. Examples. crash cushions, deformable barrier walls.
- Warning: The barrier generates an audible or visual warning. Examples: rumble strips, proximity alarms.
- Virtual: The barrier is established using signs, signals or markings and the laws which pertain to them.
A barrier may be benign for dual-track motor vehicles, yet overturn single-track vehicles. These can topple over a low guardrail or Jersey barrier. A sham barrier for dual-track vehicles such as a flex post can tangle with the pedal or leg of a bicyclist, becoming a threat barrier. Reflectorized pavement markers, which are little more than a virtual barrier for dual-track vehicles, can throw a bicyclist –see this video example.
These considerations are lost in the design of many bicycle facilities. Barriers that are hazardous to bicyclists are being used because they are normal traffic-engineering practice, sometimes only due to lack of knowledge but sometimes enforced through design standards.
On the other hand, a high railing with a handlebar rub strip can serve as an effective and safe deflection barrier for bicyclists, even though it may be too weak to contain a heavier dual-track vehicle.
In Orlando, Florida recently, I saw two other examples of misuse of barriers:
- Flex posts used ahead of and behind a parallel parking space which had been reconfigured as a bicycle parking station. Motorists parking in the next spaces would expect a light, stopping impact if they moved too far forward or back at very low speed. The colloquial expression is “kissing bumpers.” Lacking this warning, a motorist already had backed up into the flex posts and damaged one of the bike racks. Here, rigid bollards or a guardrail would be appropriate.
- Raised reflectorized pavement markers are being used on bike lane lines, neglecting the fact that bicyclists must enter and leave the bike lane, and often do best to ride along its edge. Nationally recognized guidelines specifically prohibit the use of raised markers here, for that reason.
The most common misused barrier for bicyclists is probably the low railing, which will topple a bicycle over. That is seen in many different varieties, ranging from the conventional Jersey barrier or guardrail to low wooden curbs lining boardwalks, to hand-height railings alongside paths.