设路灯的高为x米，∵GH⊥BD，AB⊥BD，
∴GH∥AB．
∴△EGH∽△EAB．
∴
| GH |
| x |
| EH |
| EB |
同理△FGH∽△FCD
| GH |
| x |
| FH |
| FD |
∴
| EH |
| EB |
| FH |
| FD |
| EH+FH |
| EB+FD |
∴
| 3 |
| EB |
| 4.5 |
| 12+4.5 |
解得EB=11米，代入①得
| 1.8 |
| x |
| 3 |
| 11 |
解得x=6.6．
故答案为：6.6．
设路灯的高为x米，| GH |
| x |
| EH |
| EB |
| GH |
| x |
| FH |
| FD |
| EH |
| EB |
| FH |
| FD |
| EH+FH |
| EB+FD |
| 3 |
| EB |
| 4.5 |
| 12+4.5 |
| 1.8 |
| x |
| 3 |
| 11 |