Street light spacing method and parking types

Spacing of street lights

The spacing is found from the luminous flux that must reach the road surface (lumen method):

$$S = \frac{F \times U_f \times M}{E \times W}$$

where $S$ = spacing between lamps (m), $F$ = luminous flux of the lamp (lumen), $U_f$ = utilisation factor (fraction of lamp flux that reaches the road; from the luminaire curves for the given mounting height, width and arrangement), $M$ = maintenance factor (0.7-0.8, allows for dust and lamp ageing), $E$ = required average illuminance (lux) and $W$ = width of the road to be lit (m).

Steps:

1. Fix the required illuminance $E$ for the road class from IS 1944/CIE/Nepal standards.
2. Select the mounting height $H$, the arrangement (single side, staggered, opposite or central) and the luminaire type.
3. Choose the lamp (wattage and lumen output $F$).
4. Read the utilisation factor $U_f$ from the manufacturer's curve for $W/H$ and the overhang.
5. Take the maintenance factor $M$.
6. Compute $S$ from the formula. For a single-side or staggered arrangement, use the full width $W$ ($S$ is then the distance between successive lamps); for an opposite arrangement, each row lights half the width, so use $W/2$ for each row.
7. Check that $S/H$ does not exceed about 3 for cut-off and 3.5 for semi-cut-off luminaires (IS 1944), and that the uniformity ratio (min/avg) is at least 0.4 on main roads (0.3 on minor roads). Adjust the lamp or height if necessary.

Example: $F = 15000$ lm, $U_f = 0.4$, $M = 0.75$, $E = 20$ lux, $W = 10$ m: $S = \dfrac{15000 \times 0.4 \times 0.75}{20 \times 10} = 22.5$ m. For a mounting height of 7.5 m, $S/H = 3$, which is acceptable.

Types of parking

Parking is classified as on-street (on the carriageway edge) and off-street (away from the carriageway).

A. On-street parking (vehicles park along the kerb):

 Parallel            45 degree           90 degree
 ______________     \ \ \ \ \          | | | | | |
 [car] [car] [car]   \_\_\_\_\_         |_|_|_|_|_|
 ========= kerb      ===== kerb          ===== kerb

B. Off-street parking:

• Surface parking lots: open areas near markets or terminals; low cost, but require land.
• Multi-storey (multi-level) parking: several floors; best use of costly land, high construction cost; ramp or mechanical access.
• Underground parking: below buildings or public squares; keeps the surface free, but is expensive.
• Mechanical or automated parking: lifts and stackers, used where land is very scarce.
• Park-and-ride facilities: at city edges, linked to public transport.

Nepal practice: on-street parking is regulated by the metropolitan/municipal offices and the Traffic Police (parallel parking on one side, paid parking zones), and off-street parking is required in new buildings by the municipal building bye-laws.