6. The combined metric/imperial height restriction warning triangle (diagram 530A) incorporated into a directional sign or the sign to diagram 818.5 (see figure 7-3 in this chapter and Appendix D in Chapter 7) is 30 stroke widths in height. The combined metric/imperial height restriction roundel (diagram 6292A) incorporated into a directional sign or a sign to diagram 818.3 or 818.4 (see figure 7-2 in this chapter and Appendix D in Chapter 7) is 30 stroke widths in diameter. The table below sets out recommended x-heights and corresponding triangle heights and roundel diameters for the signs to diagrams 818.3, 818.4 and 818.5, appropriate to various 85th percentile speeds. The dimensions in brackets apply where motorways and all-purpose dual carriageway roads have three or more lanes per carriageway. For appropriate x-heights for directional signs, see Appendix A in Local Transport Note 1/94, available from TSO or www.gov.uk/government/ publications/local-transport-notes.
| 85th percentile speed of private cars | x-height | Triangle height/roundel diameter |
|---|---|---|
| (mph) | (mm) | (mm) |
| Up to 20 | 60 | 450 |
| 21 to 30 | 80 | 600 |
| 31 to 40 | 100 | 750 |
| 41 to 50 | 120 | 900 |
| 51 to 60 | 160 | 1200 |
| 61 to 70 | 200 | 1500 |
| 70 mph speed limit | 240 (300) | 1800 (2250) |
7. 85th percentile speed measurement is dealt with in TA22 "Vehicle speed measurement on all-purpose roads" in Volume 5 of the Design Manual for Roads and Bridges (see para 1.3). The dry weather spot speed should be used.
75
