im gonna give u an easy reference, use the signs next to highway, ones that indicates how many KM left to exit Highway:
Doha Al Khor:
1- 8Km (u have to take care the sign 7KM is missing ;) )
2- 18 Km (double sided)
3- 28 Km
before any radar u'll find 2 (120) speed limit on both sides of road

al Khor- Doha:
1- 31
2- 18 (double sided)
3- 8