- When establishing design speeds, account for functional/context classification, the legal speed limit, terrain type, anticipated vehicle volumes, level of surrounding development, expense, and design consistency.
Procedure
- Determine the curve radius, superelevation, and offset distances from the lane’s center to visual obstructions.
- Calculate for different vehicle types the maximum possible lateral acceleration and braking coefficients:
| Heavy Vehicles | Element | Passenger Cars | |
|---|---|---|---|
| Maximum Lateral Acceleration | 0.35 x g | 0.7 x g | 0.8 x g |
| Braking Coefficients | 0.5 – 0.6 | — | 0.9 – 1.0 |
* Values are for dry conditions.
Assume a reaction time of 2 seconds.
| 3. Calculate the maximum possible speed (km/h) limited by lateral acceleration: | |||
| V= √(127R (Lateral Acceleration+Superelevation)) |
| 4. Using the speed calculated in Step 3, determine the safety factor: | |||
| SF=1+0.03476V-0.00004762V² |
| 5. Calculate the desirable maximum speed limited by lateral acceleration: | |||
| V_acc= √(127R((Lateral Acceleration)/(Safety Factor)+Superelevation)) |
| 6. Calculate sight distance: | |||
| SD_acc=2Rcos^(-1) ((R-O)/R) |
| 7. Assuming a safety factor of 2, set the braking coefficient (d) to half the maximum braking efficiency value. Set stopping sight distance equal to the sight distance calculated in Step 6. Solve for speed (Vsight): | |||
| SD_stop= SD_acc= (T_f V_sight)/3.6+ (V_sight² )/254d | |||
| V_sight=127d (-T_r/3.6+ √(〖(T_r/3.6)〗² + 〖4SD〗_acc/254d) |
- The maximum desirable speed for a vehicle type is whichever calculated maximum speed value (Vacc or Vsight) is less.
CONTACT:
Chris VanDyke
Research Scientist | Program Manager
chrisvandyke@uky.edu
