Turn Lane Design
Auxiliary turn lanes (left-turn and right-turn lanes) are added to intersections to remove decelerating or stopped turning vehicles from the through-traffic stream. This increases the capacity of the intersection, reduces delays, and lowers the potential for rear-end collisions.
Components of an Auxiliary Turn Lane
An auxiliary lane consists of three distinct horizontal segments:
- Entering Taper ($L_{\text{taper}}$): The transition zone where vehicles lateral into the auxiliary lane. It can be a straight-line taper or a curved transition.
- Deceleration Length ($L_{\text{decel}}$): The distance required for a vehicle to decelerate from the through-lane design speed to a stop (or to the speed of the turning maneuver) before reaching the queue of stored vehicles.
- Storage Length ($L_{\text{storage}}$): The length reserved for vehicles waiting to complete the turn.
Design and Calculations
1. Entering Taper Length ($L_{\text{taper}}$)
For a straight-line transition, the taper length is:
$$L_{\text{taper}} = W \times R$$Where:
- $W$ = width of the turn lane (typically $12\text{ ft}$)
- $R$ = taper ratio (standard ratios range from 8:1 to 15:1; e.g., $10$ indicates $10\text{ ft}$ of longitudinal length per $1\text{ ft}$ of lateral shift). A common urban standard is a $100\text{ ft}$ taper.
2. Deceleration Length ($L_{\text{decel}}$)
The deceleration length is based on vehicle braking capabilities. AASHTO Green Book Table 9-22 provides standard deceleration lengths to come to a full stop from the design speed:
| Design Speed (mph) | Deceleration Length to Stop (ft) |
|---|---|
| 30 | 160 |
| 35 | 215 |
| 40 | 275 |
| 45 | 345 |
| 50 | 425 |
| 55 | 510 |
| 60 | 605 |
Note: In constrained urban settings, part of the deceleration is assumed to occur in the through lane before entering the taper, reducing the required $L_{\text{decel}}$ on the turn lane.
3. Storage Length ($L_{\text{storage}}$)
The storage length must be sufficient to store the maximum queue of turning vehicles expected during a typical cycle.
Signalized Intersections
For signalized intersections, storage length is designed to accommodate the 95th percentile queue length to prevent lane overflow (spillback). A standard PE exam formula is:
$$L_{\text{storage}} = \left( \frac{V}{N_{\text{cycles}}} \right) \times F \times L_{\text{veh}}$$Where:
- $V$ = peak hour volume of turning vehicles (veh/h)
- $N_{\text{cycles}}$ = number of signal cycles per hour = $\frac{3600}{C}$ (where $C$ is the cycle length in seconds)
- $F$ = queue accumulation factor to account for random arrivals (typically $1.5$ to $2.0$ for $95\%$ overflow prevention; use $2.0$ if no factor is specified)
- $L_{\text{veh}}$ = design length of a single vehicle (standard passenger car value is $25\text{ ft}$; for trucks, use $40-75\text{ ft}$ depending on the truck type)
Unsignalized Intersections
At stop- or yield-controlled intersections, the storage length is based on the probability of arrivals during the peak hour, often estimated as 1.5 to 2.0 times the average number of arrivals during a 2-minute interval:
$$L_{\text{storage}} = 2.0 \times \left( \frac{V}{30} \right) \times 25\text{ ft}$$Worked Example
Design a left-turn lane for a signalized intersection approach under the following design conditions:
- Roadway design speed = $45\text{ mph}$
- Left-turn design volume ($V$) = $270\text{ veh/h}$
- Signal cycle length ($C$) = $120\text{ seconds}$
- Lane width ($W$) = $12\text{ ft}$
- Taper ratio ($R$) = 10:1
- Use a random arrival factor $F = 1.8$ and a vehicle design length $L_{\text{veh}} = 25\text{ ft}$.
Calculate:
- The entering taper length ($L_{\text{taper}}$).
- The required storage length ($L_{\text{storage}}$).
- The required deceleration length ($L_{\text{decel}}$).
- The total length of the left-turn lane.
Solution
1. Calculate Entering Taper Length ($L_{\text{taper}}$):
$$L_{\text{taper}} = W \times R = 12\text{ ft} \times 10 = 120\text{ ft}$$2. Calculate Storage Length ($L_{\text{storage}}$):
- First, find the number of cycles per hour ($N_{\text{cycles}}$): $$N_{\text{cycles}} = \frac{3600\text{ s/h}}{120\text{ s/cycle}} = 30\text{ cycles/h}$$
- Find the average number of left-turning vehicles arriving per cycle: $$\text{Average arrivals} = \frac{V}{N_{\text{cycles}}} = \frac{270\text{ veh/h}}{30\text{ cycles/h}} = 9\text{ vehicles/cycle}$$
- Calculate storage length: $$L_{\text{storage}} = 9\text{ vehicles} \times 1.8 \times 25\text{ ft/vehicle} = 405\text{ ft}$$
3. Calculate Deceleration Length ($L_{\text{decel}}$):
- From the AASHTO table for a design speed of $45\text{ mph}$: $$L_{\text{decel}} = 345\text{ ft}$$
4. Calculate Total Turn Lane Length ($L_{\text{total}}$):
$$L_{\text{total}} = L_{\text{taper}} + L_{\text{decel}} + L_{\text{storage}}$$$$L_{\text{total}} = 120\text{ ft} + 345\text{ ft} + 405\text{ ft} = 870\text{ ft}$$The designer must specify a left-turn lane with a $120\text{ ft}$ taper and a $750\text{ ft}$ full-width lane ($345\text{ ft}$ for deceleration plus $405\text{ ft}$ for storage).
References
- A Policy on Geometric Design of Highways and Streets (AASHTO Green Book), 7th Edition, 2018, Section 9.7.2.
- NCEES PE Civil Reference Handbook, Section 4.3.2.