b) On a Slope. Unlike a straight downhill path (see Paragraph 6.4.1b), a curved
downhill path is actually effective in deterring vehicle attacks. This is because the extra velocity
gained from travelling downhill can easily cause the vehicle to skid or topple. Therefore, if a
protected area has downhill approach paths, the local terrain can be modified so that a straight
driving path is impossible. Caution should be exercised when designing roads to decrease velocity.
Posting speed restrictions along the path is strongly recommended to reduce the possibility of
To determine the final velocity at the end of a curved path, use the length of the curved
path as the length in Figures 5 and 6. Figure 7 can then be used to determine the velocity at which
the vehicle will skid.
Attack Routes Parallel to the Barrier. Any path where a vehicle is forced to make an
abrupt (short radius) turn before impacting the barrier will reduce the energy transferred to the
barrier. Short radius turns can effectively reduce vehicle speed by forcing the vehicle to slow down
to avoid skidding and will reduce load transfer, if the angle of impact is less than 90 degrees to the
barrier. Therefore, the amount of energy that must be absorbed by a perimeter barrier depends on
the angle of impact (see Figure 3, perimeter roads A and B for a graphical representation of this
angle of impact) and the final speed of the vehicle at impact. The perpendicular component of the
velocity determines the load transferred to the barrier. By using Figures 8 and 9, the impact angle
directed toward the barrier, based on the offset distance (distance between restricting barriers, i.e.,
the distance between curbs or barriers that will limit the available turning radius), can be
determined. These figures are based on the formulas provided in Paragraph 6.4.2a. Figures 8 and 9
show the impact angle versus speed for a given offset distance for friction factors f = 0.5 and f = 0.9.
The curves can be used to determine the angle of impact, "θ," knowing the values of the friction
coefficient, "f," speed at the start of the turn, "v," and the offset distance available.
Once the angle of impact is determined from Figures 8 and 9, the speed component
perpendicular to the barrier, "Vp," can be calculated using Equation (7).
Vp = v sinθ
Vp = speed component perpendicular to barrier
v = speed at start of turn
θ = angle of impact